I've never been forced to do it but I do know fo a coach who got his guys to do full training sessions in head gear and wearing a mouthguard.

He coached amateurs obviously and wanted it to become normal to them to be able to train hard wearing it without it phasing them in any way.

I have to admit - even though i have a nice light, well ventilated headguard - in the Australian summer it can get hot after a few rounds with the headguard on.

You loose a lot of body heat from your heat so if you compete in warm climates maybe it's not such a bad idea to get used to it!