I don't imagine it was intended to be a 100% efficient exp source. If you're leveling a class, sure, you get full value from it, but if you're playing your 50, well, it's not intended to really do much. The Challenge Log is just another way to gain exp for leveling.

I see no reason they should alter it so you can constantly play on your 50 and then shift the challenge exp to your lower classes. You could level your classes without ever even 'playing' them, and how much sense would that make? Working as intended, and doing a good job I'd say.