The partial training was implemented so that one did not have to dump xp into
crap skills and then reinc just to earn the higher levels. It does not
particularly have anything to do with making leveling easy (although it does),
it's just there so that getting to level 100 without reincing is not brutally
horrible. If you want some spiffy reason as to why the cutoff is level 50,
take it as a small reminder of what life used to be like for those who did not
like reincing.