Much of the skill of building in trem is being able to build what is most useful given your limited resources and the current game situation. If you have enough BP to do almost everything you want, its reduced to a building race. What you're proposing is a step in this direction really. You should have to make sacrifices in base defence if you want the advantage provided that forward booster.
Forward boosters are expensive, yes, but that's balanced against their high usefulness.
Why not have an egg at a forward base?
You can't build a forward booster
without a forward egg, otherwise there is no creep... Its not a case of "is it worth putting a spawn in my outpost or not?"
humans dont really need repeators
Yes they do, or at least they are so useful you'd be daft not to at least try to make some use of them. 2 free forward buildings for 4BP is a bargain.
helps if humans destroy the original base without destroying the forward one.
Bit of a moot point. By this point chances are you're screwed anyway.