In a typical NL or AL-only league you're still only evaluating about 250 players. It's not that much more work than a mixed league. But now you're trying to figure out who's getting playing time and who's not which imo is not what fantasy baseball is about. In a perfect fantasy world, everyone gets equal playing time and those who perform better should be worth more. In mixed leagues you don't have to worry about playing time as much.
In my 16-team, 23-man roster league, you have to evaluate about 370 players so that takes skill there or at least determination.
"And so he spoke, and so he spoke, that lord of Castamere. But now the rains weep o'er his hall, with no one there to hear." - The Rains of Castamere