My understanding of the DPSK limit is that it means that a single user (username) can only have "x" devices maximum attached to the WLAN under their username. One use is to stop people from sharing their username and password with others and giving them access to the network under their username.
In my school, I set a limit to 4 devices per user, and if there is any specific need for a student to have more than 4, then I temporarily turn it off, create the dPSK and turn it on again.
More often than not a student will say "But I don't have that device any longer" in which case I just delete the DPSK entry for the old device which enables them another slot in their limit of "x" devices :)
Hope this helps.