rear brake is rarely used to actually brake,more to help keep the front under control,and (if your good enough) to help tighten up a turn,obviously the riders up at the sharper end of racing are using it more,but generally there are more important/useful things you can do(or at least should be) before the extra parameter of the rear brake comes into effect.
the video mentioned i think is of a lap by Danilo petruci,showing how much he uses it,it's alot but theres no point comparing a MGP riders style with the general trackdayer.