I've been on a lot of flights through DFW/ORD that continue on to somewhere else. I've frequently heard an announcement stating that passengers continuing on to XYZ are welcome to stay on board; I've never heard an announcement stating that they must deplane.

I would also think that being on a thru flight would significantly improve the chances of you making your connection if YYZ-LGA is delayed. Last Friday, I was on a DFW-BOS flight that was a continuation of a FAT-DFW flight. FAT-DFW was about 2 hours late getting off the ground, and DFW-BOS was delayed accordingly... and this was at DFW, where they certainly have some spare MD-80s around. (In fact, we ended up taking a different MD-80 to BOS, as they discovered that they needed to take the FAT-DFW aircraft out of service after it arrived.) The chances of them having a spare 738 lying around LGA are slim to none, so as long as YYZ-LGA makes it to LGA eventually, I would expect that you'd make your connection to MIA.