I've heard of operations that had large trough's of dirt (Sunshine #4) that they would just plug a small plant into and use the dirt for two or three cycles. At harvest, they would just pull the plant right up out of this trough by the trunk and then plug another plant into its slot.
Personally, I've had plants that were vegging fail so I just pulled them out and after a while decided to reuse the soil. I was using 20 gallon tubs filled with Sunshine and didn't want to loose what had been practically unused dirt. Well, the plants turned out ok and really produced (like they always do for me
) but I got this really strange fungus growing in them. It was large, neon yellow banana shaped growths started poking out from the surface of the soil. Hmmmmm, but, on the whole, I toss the dirt after each grow. I plan on experimenting with adding it to my lawn to help fill in some holes out there next spring.