Characters in DARK SUN tend to be personally very powerful, with ability scores that average better than characters from other game settings, higher levels, and extra class features. In contrast, their equipment is woefully inadequate. Thus, while your character might have a high Strength score that translates into an extra +1 bonus on attack rolls and some more damage, the use of a weak bone or stone sword could cut into that with a similar penalty (plus the chance for the weapon to break). The lack of heavy armor also means that combats are faster, because attacks so rarely miss. The extra hit points garnered from a higher-than-average Constitution score really come into play for combatants who would be wearing mail or plate armor in other campaign settings; on Athas, your fighter's lucky to have a suit of bone-studded leather. Even the higher Dexterity scores usually don't make up this difference. Thus, your fighters and gladiators rely more on their ability to take punishment than on their ability to avoid solid hits -- which translates into making the advantage of the half-giant (in the 2nd edition version of the game) even more formidable, with a huge bonus to hit points.
Coming back to that -1 or -2 from your weapon's poor quality, having a metal weapon becomes a big deal when it doesn't have those modifiers. You're getting the full advantage of your gladiator's 20 Strength score! Numerically, getting rid of a -1 is equivalent in most ways to adding a +1, so getting your metal weapon in DARK SUN is the equivalent of getting a magical weapon in another game setting. This is underscored by the fact that many Athasian monsters are susceptible to damage from metal weapons but not from other materials, much like the category in early AD&D of monsters that you can hurt only with magic weapons.
Acquiring metal weapons in DARK SUN, then, is much like gaining magic weapons in other settings. A metal weapon is not only monetarily valuable, it's a treasure of great utility. Fighters will go out of their way to learn to use a weapon just because a metal version is available, because that steel short sword might not hit as hard as a bone battle axe, but it'll hit more often and it won't break. Gladiators, of course, have the advantage that they can pick up any weapon and use it without penalties at all!
This comparison means that metal weapon will generally appear fairly early in a Dark Sun campaign. Playing out battles with suboptimal gear is a feature of early character hazards, but within a level or two the party will start finding the weapons that will give them baseline performance without constantly worrying about gear breakage and ugly penalties on attack rolls and damage.
This comparison also means that at about the same time, the party should start seeing magical weapons -- but the first magical weapons they find will be made of suboptimal materials. A +1 bone short sword is functionally almost identical to a normal steel short sword; the main difference is that the former detects as magical. Thus, both of these weapons could appear in treasure hoards at about the same time. As the party gains higher levels, they'll start to see really great treasures, the +2 and +3 steel weapons, but those will always lag a little behind where you'd expect to see them in other campaigns, because the innate capabilities of the characters substitute for having the top of the line gear.
Armor's a bit of a dodgier problem, simply because wearing metal armor in Dark Sun typically means a quick route to heatstroke and inability to fight anything well. Thus, you're often stuck wearing partial metal armor (scale armor on one leg and one arm gladiator-style!) or looking for enchanted nonmetal armor. This interestingly means that those suits of +1 studded leather are suddenly really valuable because your party's fighters, gladiators, rangers, and clerics are all looking to use them! Once again, the trade-off here becomes a materials issue. A suit of chain mail (AC 5) compared with a suit of +1 leather armor (AC 7) -- the leather doesn't offer as much protection, but it doesn't include the potential problem of heat stroke. The chain mail allows you to do stand-up fighting with an expectation that some enemies will actually miss, but it will hurt your own fighting ability. This means that players are encouraged to pick their armor on the basis of specific strengths: A character with a really high Constitution score is more likely to go for the chain mail, as it may be possible to avoid the heat stroke for a while (with the right proficiencies, anyway), while the leather will be more popular among clerics and (of course) thieves and rangers. Naturally, all of this is moot if the characters are so resource-hungry that they don't have a choice! You wear that mail sleeve because it's better than no armor at all.
Another great advantage baked into the setting with respect to metal arms and armor is that it's noticeable. If your character in, say, the world of Greyhawk is carrying a magical sword that is sheathed, who knows or cares? You're just another dude with a sword in a scabbard. But if your character on Athas has an iron scale hauberk, or a sword with a metal pommel, people will pay attention to that. It's a mark of your character's expertise and importance. When your fighter takes to the field of battle in that battered steel helm, casually holding a broken-tipped steel sword and wearing bronze greaves, everyone knows -- that guy's in charge of the army! That, in turn, means more hooks as people try to inveigle the character into their dramas, perhaps to curry favor, or perhaps just to steal your steel!
(Incidentally, this was one of the minor but annoying points that bothered me in the Dark Sun novel City Under the Sand: At one point, a character arrives at a city gate on a fast-moving cart full of metal items that have been scavenged from an ancient city, and throws open the back of the cart so that the metal items fly out all over the place. This is a central conceit of the plot: The main character can sense metal, and was sent specifically to find this metal and bring it back. In this scene, metal winds up flying everywhere and nobody cares.)
If you want to underscore this quality of metal weapons in your DARK SUN campaign, you can use the same tricks that many DMs use with magical weapons in other games. Give the items their own sense of character. A steel long sword can be "a three-foot flatchet with a jagged tip and many notches along the blade, the metal burnished to a dull glow with evidence of the years of sand that have blown over it. The pommel's leather wrap has been replaced many times, and it has the feel of crimped, weathered leather that has fitted to many hands." A bronze-tipped spear becomes "A shaft made of slightly-curved bone, clearly not the original haft to this weapon, but a replacement made with the best materials one could find at the time. Sinew binds the foot-long bronze tip to the end of the bone. The knife-like blade is aged and has a green undercurrent from the touch of blood and other ichors, most pronounced near the guards, one of which is broken about a half-inch out." You can underscore this by giving the weapons specific histories (which also gives bards something to do) and even slight modifiers to their usual qualities. The aforementioned bronze spear, for instance, might not be quite as good as a steel weapon, but better than any of the other substandard materials around -- say, no modifier to hit, but -1 to damage. An iron dagger might have no penalty to attack and damage, but still have a chance to break. All of them could have a name and a history, stretching back ages into the lost times when they were made on a more verdant Athas, before they were handed down or dug up in the brutal world of the DARK SUN.