I think exams can be a useful way to get some students to work... others (like me!) would run a mile if my teacher tried to insist! When I was a kid I took and passed the whole set of grade exams because I was thinking about a career in music. Then I bit the bullet and realised it'd never make me rich or secure and I probably wasn't good enough to match my aspirations. So, if you like to think of it in the long term all those exams were pointless and all the repertoire I learned when I was a kid was exam repertoire and I didn't start playing for fun till I joined orchestras at university and discovered there was more to it than exams... on the other hand I suppose you might argue that had I not been a world-class cynic at the age of 18 and gone for the music career, I might have got the breaks and made it and then I'd be saying the exams had helped...
Anyway, nowadays, when I'm learning piano for my own entertainment and enlightenment... I don't want to do exams. They're expensive, will have no practical value for me, I'm motivated to work without them, and I want to play music I like not music the exam board sets. Life's too short to let other people call the tune.