With all due respect, we just had that in All Pro Football 2K8 (maybe not exactly what you're talking about), and this brought about some negative items on my end:
1) Generic players were always inferior, and sometimes outright dog****.
2) I felt like this crippled any chance of having a legitimate franchise mode since ages can't be accounted for anymore, though I think how Super Mega Baseball 4 handles it is they give that player their prime age of whatever version of them they're referring to (like an older David Ortiz vs. a younger one, etc.), so while it's possible I felt like this was a crippling effect... plus eventually in franchise you just end up only having dog**** generic players if they last long enough.
If they could remedy those things, I suppose it's all possible. I'd prefer to go fully generic instead though, especially if customizable but even if not. I would love to learn my own guys and the league that way, just as I do in SMB (and those names carry into future installments as well).