Oh yes, that's an argument that could run and run, isn't it? Our Catholic comprehensive here, is very strict with it's policies though, and in particular our Parish Priest is even more so. He's very 'old school' and goes very much on church attendance etc. There is a lot of paperwork that you have to submit to the Catholic comp when you apply, but the one that makes or breaks it is the Priests reference form. One of DD's friends was turned down by the Catholic comp, because the Priest had made an error, and signed her form as 'not attending.' The girl had been attending regularly with either of her parents, she's attending the catholic primary, has been baptised, done her Holy Communion etc. As you can imagine, her parents were incensed when that happened particularly as she was allocated our catchment comp by the LEA which is really poor. The situation has now been resolved, and she's been given a place at the Catholic comp.
However, this Catholic comp in particular, doesn't exclude children of non religious parents, if there are places available after all the higher categories have been allocated, then they will get in. Perhaps more so with the primary though. Our Catholic primary has children attending whose parents aren't Catholic and don't go to any church. Still, not many though. This year, the Catholic secondary's cut off was category 7 which I think is 'other denominations.'
What annoys me about all that, is that children who attend catholic primaries, been baptised and Holy Communion-ed, ticked all the boxes etc, still have to bend over backwards (well, the parents do) to get into the Catholic secondary. It's very Dickensian.