Ive seen some that keep the blowers on a few seconds after the call is terminated. But you are right if they had a post purge they would last forever.
York has this funny new thing now. I find on some of their units the time delay relays will start to act up and the unit will actually trip the high limit before the blower comes on. Instead of just giving you a replacement TDR, they try and shove a time delay control board down your throat. I installed one. It actually tells you to bypass the connections for the combustion fan motor so that it operates off the heating relay.
Basically before it worked that once the heating relay opened at the end of the call, the TDR would keep the comb. motor energized until the TDR opened. This was the post purge. But since these new boards dont have that option, it comes on and off with the heating relay.
Sounds like they want to sell more heat exchangers.