Well, I think it should be imperative for a country like Pakistan to play their starting batting line-up for numerous reasons (to become accustomed with each other, role in the side (stability etc), running btw wickets etc etc) and unlike the bowlers, there is less risk of injury (atleast serious injury).
These practise matches are essentially extended training sessions, but out in the middle.
I'm not too fussed with Aamer, Gul not playing (because they are bowlers, injury concerns etc), nor do I particularly care about Asif's performance (because he is a seasoned Test cricketer, and understands the constraints of the game, so is likely to restrain himself and 'show up' at the first Test), however, I see no reason why batsmen (new to the format; Afridi, those who are likely to be relied on heavily; Akmals) aren't playing, especially seeing how poor our batting performance has been in Tests for over a year.