So, under this system, I've got BLM at 50. I want to try PLD and MNK. So I rush GLA to 50 without picking up any cross-class skills just so I can get the extra boost on PGL, and rush that to 50 without any other cross-class skills, just so I can get even more boost when I get around to cross classes.

Which is why they did away with the scaling % bonus to begin with and made it so that you get the flat 50% as soon as you're leveling a class that's any lower than your max level at all. You start with 50% as soon as you can switch classes at level 10, and it stays 50% the whole way as long as you have any level difference at all. That way, players aren't "punished" for stopping at 30 to get their job and level cross-class skills and such.

And leveling is stupid fast no matter what you do.