They're still edible after cooking, but they take on a texture people don't usually like too much. It's kinda similar to them being overripe, minus the browning, very soft and maybe even a little slimy. The flavor changes a bit too, but I'm not sure how to describe it. It may be subjective (I'm just completely used to eating them fresh) but it seems like they're definitely worse cooked. In any case, they don't need to be cooked, so why bother?
Note that you can heat them a bit without those effects, though, for example the brief cooking of quesadillas will leave them pretty much fresh.