I've had enough bad experiences with cotton that I wouldn't take any at all. However, cotton knit does have its place for desert hiking in hot weather--get a T-shirt sopping wet and let it dry on you. That assumes there are water sources, of course!

Cotton, even in a 50/50 or 60/40 blend, absorbs lots of water, gets horribly heavy when wet and takes forever to dry. When it's hot and the humidity is high, cotton gets soaked with perspiration which doesn't evaporate. A good wicking synthetic will be more comfortable. Check the athletic departments of Target, KMart or WallyWorld for alternatives that are less expensive than but just as good as the expensive stuff you get at REI. Wicking shirts, running shorts, nylon track pants. Some like to head for the nearest thrift store for oversized silk dress shirts to wear for hot weather hiking.