I'm one of those people who often uses Wesker (Midnight). Personally it annoys me when my partner is constantly on my hip because, in my experience, its easier to lose combos that way, especially on levels like ship deck and the missile facility because enemies tend to spawn slower and it helps to be a fair distance away from each other.
That being said, I do like the Wesker/Jill pairing in Mercenaries because you can get some nice combos with Jill's handgun alone and Wesker can take down tougher opponents with either magnum or the Hydra, depending on which costume one chooses. I just don't really like to be joined at the hip on levels unless one or both characters are in dying status or one of the characters is not yet skilled enough to get through the level without dying.