I personally don't think we (YACoin) should force people to keep their clients open all the time. And I am at a complete loss how POS is supposed to be some kind of security utopia, especially when POS it nearly killed us. We neutered POS at block 420000, so It's behaving itself nicely now.
From what I have read, the problem YAC had with POS is YAC (and its offsprings) specific. POS promises to offer solutions to problems with POW that will probably have pure POW coins killed. POS is very complex, however.