I really thought about this for quite a while, because it struck my mind in Rollcage as well.
I answered "No", because the idea of having to consider whether to drop or keep a powerup is quite important, imho.
But, being able to enhance that powerup with another pickup would make this consideration even more important.
Think of this: Leading the race and picking up the gattling will likely result in the player firing it into the blue. But being able to enhance it with another pickup might make it interesting to keep - Twice the gunpower when second place overtakes you
This will be placed in suggestions as well.