I also think that fixing a flat once in a while just isn't of a huge deal. Fully loaded or not doesn't make that much difference with regard to the level of difficulty, for me and with my bike at least. The only draw back to my loaded touring bike in that regard is that I have to pull the skewer out to get the front wheel off, a minor annoyance. Otherwise I actually find it convenient that when I lay the bike on it's side, the panniers make the bike lay solidly and with the wheels well off the ground.

I would expect relative ease of flat repair for folks using two legged kick stands as well.

I guess that some folks have panniers that are mounted in a way that doesn't allow access to the quick release levers. The old style bungee cord systems like I use allow easy access.