I'm a fan of classification seniority. Here's why:
At NW, I had 3 dates on the ramp: PT (also my DOH), FT, and Lead. For FT, anyone with an earlier DOH could've bid it. They didn't. For Lead, anyone with an earlier FT date could've bid it & had it. They didn't.
So let's flash forward a several years: I've slogged my way through all the sh*t shifts, terrible days off, etc. and finally claw my way to a (relatively) decent spot. How is it okay that someone that passed on the position(s) the first time around now takes it, and cuts in line? It's not.
At least with classification time, people can move around, but those that are already there are not usually negatively impacted for shift bidding.
Just my .02...