Delta, on the otherhand, seems to pop up every now and then in the MR forum with some great deals out of SEA.

I flew into SEA last year to do a SEA-SNA MR on Delta (credited to AS) and there was another cheap fare recently posted out of SEA on DL.

I agree that AS doesn't seem to have many low fares out of SEA (or GEG, for that matter). But if you're making MVP or MVPG using partner travel, it's worthwhile to keep an eye out for DL fares.