To the best of my knowledge, there are two kinds of PUP, active and reserved.
With the active-PUP, a player can come off that list at any time. The roster spot does count against the 80-man roster as well as the salary cap.
With the reserved-PUP, a player cannot come off that list till after the sixth week of the season. It does not count against the 80-man roster (or 53-man in-season roster.) After the sixth week the player either needs to be activated or go on Injured Reserve.
A player can only be placed on PUP at the start of training camp. In theory, it is a designation for players who failed their physical. They still get paid and can attend team meetings but are not allowed to go on the field and practice.
I think it's more of an insurance for a team that would like to prevent a potential injury settlement if a player that is not 100% gets injured in training camp. I don't think it's a sneaky way to get around the 80-man roster limit, and I doubt any team would let one of these guys practice since these practices are open to the media and public.