I've read about this theory since "Happily Ever After". Well Desmond is acting very Jacob-like. He has become creepy both on and off island and nearly killing Locke. He has become very self serving so yes I do see your point.
As for the letting him talk to him that just seems to open up a can of worms trying to figure out who let who talk to who. Everyone who has dealt with both let them talk so I would assume they are all their pawns.
I would hate that Desmond could "become" Jacob. I don't like the idea of either Jacob or MIB taking over a character and supplanting them with their "souls" or whatever. I am not a big fan of the theory that we will see a scene like the one we got of Jacob and MIB in last season's finale only with two of the Losties representing them. That to me would reveal that they lost and were not able to defeat Jacob and MIB.