You can reuse soiless mix but its a process and it takes work.it has to be flushed,cooked and what not if you had thousands of gallons of soil it would b worth doin.Or had multiple tubs in different stages in your basement. But just to pull a plant shake off the dirt and throw another in ur askin for trouble.if you got a small grow just buy a bail or two.
I use the same soil in my mini grow to flower and re-veg. Learned from DrBudGreengenes thread that soil can re-used like this several times. I read that each of his ladies get re-vegged 3-5 times each in the same container and same soil. I don't remember if that is in the thread I linked or the one at ICMag.