As already mentioned it doesn't matter what system you use - as long as you are doing it well. The worrying thing is not that we are persisting with zonal marking, but that despite us being vulnerable from set pieces for a while now, it doesn't seem to be improving.
A few seasons ago we had a really good defensive record, but now every time a team get a corner or a free kick near goal against us it's a worry.
I don't know what is done in training to practice these things, but I would be happier to see someone go on the posts at least to minimise the dangers.