Suppose that I really covet a certain player.
Say Matt Morris in last year's monthly FA draft. The guy picking in the #1 slot of the monthly FA doesn't like him and would be more than happy to trade the pick for Jose Cruz. However; he can't trade the pick so he ends up picking another player. So the guy with the #3 pick ends up taking Morris. This guy thinks that Morris is a stud and Cruz stinks. So he won't trade Morris for Cruz.

The same principles would apply if you couldn't trade picks in the rookie drafts.

This is an example where being able to trade those monthly FA picks is very useful.