Definitely coaches preference. At our gym some girls use them and some do not, in about levels 5 through 8. Our coach doesn't mind as long as the girls don't mess around with them constantly.
I was at the Gynix International this weekend and some Moms from Omega Gym club were seated next to me. They said at their club, fantastic elite level gym in Vancouver, Canada, nobody wears
grips. They have Russian coaches who just do not think they are needed. Their gymnasts are great bar workers, and obviously aren't struggling without grips.
It really makes me wonder when I see the Chinese, Russian and Romanian girls do fabulous bars without grips. The amount of time breaking in, getting used to and adjusting grips seems a lot of time wasted.
My girls do not wear grips and I am not sure what I think about it. Oldest has tried them, but seems to like it without better. Hands seems to rip with or without grips, I have been focussing more on the skin condition this year and we have had no real rips. My girls ice any red or blistered bits, then we apply really good moisturizer and sleep in cotton gloves. In the bath they rub off the callouses a bit so there is less to rip off on the bars. This has worked well up to now.
Of course many girls just want to have grips, 'cos they look cool, which is fine if your coach agrees.

But, I still remember the girl at DD's last comp who forgot her grips, her normally fabulous bars were a mess.
The moral of the story is, you can forget your grips for the big meet, but you cannot forget your HANDS!!!