Drow items don't decay in sunlight. It's bad game design, and really only existed in the first place to screw the players (in the "look at all this cool stuff you can't use" way of things).
Consider these two scenarios for Drow Decay Syndrome (DDS).
1) Because the item decays in sunlight, it costs less to make than a more permanent item. So a drow that should have 10,000 gp of gear actually has (for example) 12,000 gp of gear (if computed without the discount). So the drow is more of a challenge than another creature of its CR. But when the heroes defeat the drow, they can't take his treasure home, so they get less treasure than they should.
Net result: Good for the monster, bad for the players. Not fair, and this not a good idea.
2) Because the item decays in sunlight, it costs more to make because your enemies can't use it against you. So, a drow that should have 10,000 gp of gear actually has (for example) 8,000 gp of gear (if computed without the price bump). So, the drow is less of a challenge than another creature of its CR. And when the heroes defeat the drow, they can't take his treasure home, so they get less treasure than they should.
Net result: Bad for the monster, bad for the players. Bad all around, and thus a bad idea.
That's why in 3E drow items don't decay in sunlight. FYI, this isn't just my opinion; Jonathan Tween, Ed Stark, Rich Baker, and I had a discussion about this during the design of the FRCS, and we agreed with the above reasoning.
Question: How is this different than having encounters with monsters that don't have treasure? Shouldn't the awarding of treasure balance out over time and make up for the DDS equipment?
Answer: It is different because in most circumstances you're not going to have an extensive campaign against monsters that have no treasure, whereas with drow quite often the scenario is a long foray into the drow tunnels, which means many encounters in a row (even extending over several game sessions) where the heroes get little or no treasure. True, they can use the drow equipment while on such a campaign, but when they leave the underground tunnels laden with "treasure" that's going to evaporate, that means they're not being fairly rewarded for their work. If the drow are just an occassional opponent it's not so bad (they would count as a no-treasure monster in those circumstances), but you still have to keep in mind how the cost of DDS (however you interpret it, using option 1 or 2 above) affects the EL of the encounter.
Question: How is this different than giving unholy weapons to the enemy PCs of an all-good good party? They can't use their enemies' weapons, which is what you seem to be arguing against.
Answer: This is related to the above answer ... unless you're running a very specialized campaign, you're not going to have a long series of adventures where the best gear the enemy has is unusable by the heroes. As an occassional annoyance it is acceptable (like a cursed item), but if it becomes routine it means the heroes aren't being properly rewarded. And even if the heroes can't use those unholy weapons, they probably know a non-good ally who can use them, or can at least sell them to a neutral party and get something for their trouble (to a neutral character, a +1 unholy sword is still a +1 sword, even if he never uses it to strike a good opponent). For example, it would do well in the hands of a follower of St. Cuthbert, a neutral god who nonetheless fights evil.