I paid for my bridesmaids, but not out of a sense of obligation. I DEFINITELY wanted them all to have at least their hair professionally done, but due to the early morning wedding, the only option was in-room service. I was afraid some would balk at the cost.
So, my options were: don't care if some have pro hair and some not OR pay for it myself. In the interest of uniformity, I paid for it.
Also, I really wanted us to get ready together, and this ensured that. It was an awesome time and some of the best money spent!