What we have now was probably the first/quickest solution that popped.
What you've got is how every MMO on the market that has Qs for PVE and PVP do it. I know it seems to make your arguement more valid (even tho it really doesn't) to make it seem like they were simply to lazy to come up with the Q but not really in the Q idea of stacking. Under this solution you'd still take forever to get a FP Q to pop cause PVP pops are dime a dozen and you'd have a cloud of perpetually stuck people in the limbo in-between the two. Dan is #19 and gets out of his PVP Q and gets another 30 seconds later, so he has a whole 30 seconds for the PVE Q to pop before he's back in limbo again.
How is this in anyway a workable solution? How much of the games resources are going to be tied up keeping track of the massive cloud of people in the Q but not really because the are in a PVP match at the same time? Yes eventually they would get lucky and get a pop inbetween the two but in the meantime the only reliable way to get in a PVE grp is too...not Q up for another WZ...just like they are asking you to do now.