Dont forget that to find a POS block, one must keep his coin without moving them for a month before trying to find some block. That is some kind of cost that we may have to consider.

I think that if we want to be able to guaranteed "interest" (POS) we have not a lot a choice aside to priorise POS block. If not, one may have to wait 3 month or even more before being able to move their coins if they dont want to loose all the interest related to those coins that have been sitting there for that long...

I may be wrong, but I think most if not all POS coins are designed this way.