Trouble is generally people don't take very good care of rental anything. Thus, you may loose the value of the dress after one or two rentals - which would hardly cover it.
Hasn't been my experience with renting dresses. The ladies who rented mine but didn't buy them were very careful with them. Local dance community is not that big, and if someone rented a dress and wasn't careful with it, people would hear about it and wouldn't rent to that person, I suppose. I certainly would have told everyone and especially the owner of the studio who organizes the dress try-on, that so-and-so messed up my dress if it happened. Besides, a dress loses value every time you wear it anyway, and unless you got a resale when it reached a "steal" status, you'd never be able to sell it for what you paid for it.