Pre-paying the gratuities is nice because you don't have to worry about it on the ship. They are paid for and that's it. However, I think requiring them to be pre-paid is just so RCI and Celebrity can get that money in advance. The Carnival lines all have anytime dining as well, and you don't have to pre-pay, they just charge you the gratuities either on a daily or weekly basis, and distribute them into the tipping pool for the various staff you see each week.

We generally prefer fixed seating, and always had good luck with table mates (hopefully those table mates also felt like they had good luck LOL). The one time we had anytime dining, on Princess, we were something like 400 on the first seating waiting list, which struck me as funny that there would be such a wait list when they kept trying to tell us that anytime dining was so popular. However, we found that on the nights we showed up at the dining room at the same time as first seating, we got seated right away. The couple nights we went later, say 7pm, we ended up with quite a wait.