I'm a bride in a few weeks time, and while I have bought all accessories for the adult bridesmaids they are getting their own dresses. All I told them when I asked them was it had to be a certain colour (I didnt care if they werent exactly the same shade) in whatever budget/style they wanted, and that they would be able to wear again.
I also made it clear that I would understand if they couldn't, and to feel no obligation to accept. Both were fine. I got the children's outfits, partly because one is DSD and the other is my god daughter and we have shared finances. I think it depends on what you want/ what your bridesmaid want and if you discuss it before hand.
Everyone's different. If I'd picked out a specific dress and told them to buy it, I think that would have been wrong. If any bride expects the bridal party to pay, then the bridal party need to have total choice over the items xx