I considered using my "pluses" for DQ or PI but I compared the cost of a waterpark admission ($36.21/$29.82 adult/child) to the cost of discounted passes for PI ($16 w/o the MouseSavers discount) or DQ ($24/$20 w/o MouseSavers discount), it made more sense to save my "pluses" for the waterparks.
Of course, if you don't have the no-expiry option added to your passes, it would make better sense to use up all the pluses before the passes expire, regardless of what you choose to use them on.