Well I'm glad there are other people out there that care about this. It's always been something about Low-cost carriers that has appalled me from a safety perspective.
Nowadays the only LCC that I will buy tickets on is Air Asia (the others are just too crap and dangerous). But I've obviously become too conditioned. On a recent trip to Bangkok I paid for a front bulkhead seat, on the return leg (QZ7717 15 AUG 10 BKK-CGK) I was in the second exit row and noticed exactly what you described.
The first time I encountered paid exit row seating was when I was booked on Valuair after they started doing it and I was appalled that on an otherwise full aircraft the exit rows could be empty with no-one to operate the over-wing exit in an emergency.
I hope Air Asia sort this out because they are the only LCC I am now willing to fly with but Tangan you are absolutely correct - this practise needs to stop.