You could argue you don't want to sink too much into Leaf, because he's actually not that great at combat as he could be and plus he's got low move and you'll just end up wanting a boss killer like Azel and then Warp bots.
FE8, FE4, FE2, and presumably FE13 are the only games where you can even use everyone, simply because they have either grinding areas or the arena for 4 and give out enough exp for the whole party. FE2 is more of you can use everyone, but half of them are shit.