The common argument is it stops arguments. you let kids use them during breaks and they say "Oh well, you let us use them during breaks, we should be allowed to use them in class".
And if you tink a simple no is going to stop that argument in any school, you're severely mistaken. no matter what control your staff has, they will always resort back to that argument. so its better to nip the bud before it blooms.
Also, you know there are people who wil use them, and continue to use them once they're in class under the excuse "I started it in my off time i have to finish now" and they'll fight ofr that and make themselves the victim in the eyes of their parents and class mates if they're told to stop and action is taken.