You only pay for mIRC once, whereas if you have NT and go to a shop to ask if you can have XP for free they'll laugh you right out of the store. Hence different products.
So far the only real negative things people can actually come up with in 6.1x are a few minor GUI changes. Sorry but I find that to be a completely ridiculous reason for people to not upgrade. The integral check and the About/Registration dialog appearing are both moot points seeing as they only affect people who are breaking the licence agreement.