I think the 50% rule actually has two related but separate goals, and it is the one place in the course where I think it is sometimes more important to follow the spirit of the rule, rather than the letter.

I can't speak for art so much, but learning other things I often see people who get stuck doing exercises and studies indefinitely as they wait for the real world applications to become easy enough. E.g. I've seen language learners spend literal years just cramming vocab and grammar, and when you tell them they need to pick up a book, watch a tv program or talk to someone they say "I tried but it's still too hard. I think I just don't know enough yet" and often they never reach that goal despite putting in hundreds or even thousands of hours. The fact is, some things are just hard at first and if students wait until they feel ready many will never leave the start line. Also, isolated exercises have a synergistic relationship with real world practice that you lose when limiting yourself to just one or the other. I'm as guilty as anyone of falling in to this trap, so I find it helpful to remind myself this is the same impulse that told me not to start an art course in the first place, since I couldn't imagine myself ever being good at it.

So that's why I think the 50% rule has the restrictions it does, but fun is also an important part of it, and it could even be argued that avoiding things you like because they don't fit the rule's criteria is breaking the rule, since part of the rule is that you make the art you want to make irregardless of how useful it is. This is why I say the spirit is what's important. If figure drawing is fun but drawing from imagination feels like pulling teeth, then do both and treat the first as fun time and the latter as a necessary exercise. Or count them as contributing some portion of time to each. Or don't worry too much about the specific timings and just make sure you have a full spread of activities. I prefer to think of things on a spectrum from "purely for fun" (e.g. an abstract doodle) all the way to "purely educational" (e.g. linework exercises) rather than trying to fit everything in to binary boxes.

That's my opinion anyway. I must admit I'm a bit biased by the fact that I suck at time management, but I also find it a significant drain to be constantly trying to categorise every activity.