Why would you use your hands to avoid fire coral? I'd much rather do a quick back kick or better yet, stay far enough away from the stuff that I don't have to worry about it.
Diving is full of rules, but what it comes down to is that it is an intelligent sport. To dive well, one needs to engage their brain. What is the 'smarter' way to deal with stopping forward motion/adjusting position in the water? With your hands which do not make good fins or with the big paddles attached to your feet? Basically, there is never a reason to fin with your hands unless you have not learned how to do otherwise, in which case it is a problem that can be solved with practice unless you prefer looking like you are doing a doggy paddle in the water.