Decapping them does allow the water to drain out easier so that they dry quicker.
For rifle brass, I often wet tumble for 15 minutes without the pins, just to get the outside clean. Then I dry just enough so I can lube and resize/decap, then back into the tumbler WITH the pins for the final cleaning. This gets all the lube off as well.
I have wet tumbled pistol brass a couple of times without decapping. After the brass was all dried, I had several "ringers" where the primers came apart, some of them still stuck in the primer pocket. I don't know if if was because of the wet tumbling, but that was the only time I ever had it happen.