I think grades are a great way to learn and understand your instrument as well as your role within a band. Many grading bodies also offer formal qualifications that count towards UCAS as well as other things. I don’t however feel that learning grades is enough for anybody to learn on it’s own. If you spend all your practice playing the same things over and over again it will get to the point where you don’t want to go and practice because there is no variation and nothing new to get your teeth into.
Therefor I feel it is vital that if you do learn grades for whatever instrument you play then use it as an extra addon and make sure you have other things to work on. It might be that you are in a band and you are working out new parts for that, or listening to your favorite songs and trying to work out how to play them, whatever it is keep the new things to practice coming to help avoid getting bogged down with the same old stuff. That way you can work through your grades and develop that way as well as in other ways which will make you a well rounded player.
I would love to hear from you if you are learning grades and how you motivate yourself to practice and also what other stuff do you practice to avoid getting bogged down?