I have the same problem with using microfleece in BumGs - because there are two layers of staydry fabric for the wee to go through, if he had a big wee it wouldn't absorb quickly enough and would run straight out the sides or back.

But I don't have the problem with flushable liners (I can't remember if mine are Eenees, but they are the papery kind).

And no, it's not really necessary to line BumGs, but then, I have a little squirt. I found liners pretty useless at any form of poo catching, so only bother if I need to use a barrier cream.