Not only in the bowling department, but it may be Rehman's batting which is giving him more value over Saeed Ajmal.
If Rehman can bowl 10 overs for 40 to 50 runs, and hit a few runs quick at the end, or help chase runs at the end, it may be very well worth him playing.
If you really look at Pakistan's batting line-up, they bat till #10 with Rehman, as Gul can also swing and hit the ball a long way.
It is a defensive option for sure though, because the team management are thinking we need batsman of some sort at #10, but we are prone to collapses, and historically our bowling has usually always taken care of itself even if weakened, it is the batting that needs to be strengthened.
Not that I necessarily agree with this ideology, I think it is the team's thinking.