Without loss as well as gains there's no balance. That inherantly improves the higher levels over lower levels further than the abilities they train, and I wouldn't agree with moving in that direction.
The point about what stats are lost is important; however, I think an amicable tie could be found. In the instance of tinkers, if X dexterity is gained, X intelligence is lost. If X strength is gained, X reactions is lost. So we're clear as to what I'm thinking, X should be 0, 1, 2, or at the high end, 3. Assuming 10 levels per check for loss, at level 60 overall you could've lost nearly 18 to one stat (at the very most, assuming the same stats are modified and the maximum change is given) and gained the same 18 to some other stat. I don't think that would border on "detrimental" personally, but maybe the neat factor is not worth the potential danger for players bent on optimized play.
