Absolutely TFS - the issue is whether instructors get paid for ALL their time with a student (but not including the pub - I draw the line at THAT!)
I had 3 excellent instructors (all with same school) for my PPL who did full brief and de-brief every time. Not sure how they were paid; I think a couple were salaried.
But, to me, briefings are not the same as groundschool for the PPL syllabus. So compulsory groundschool may not achieve your aim, which is one of fairer pay as a means to getting better quality briefings.
Cheers
Whirlygig