I just asked them if they were sure and explained how it keeps runners from pulling back packs of other mobs. I wasn't totally 100% certain I was the correct one -- it made no sense that it would effect the party, and the tool-tip said nothing about it, but in FFXI I could see a positive having a negative trade-off like that and I'm far from confident in my WoW knowledge... I was 99.9999% sure I was right, but just not enough to argue back. On top of that, after the day I had, I worried they'd kick me if I didn't listen. (I also put my pet on passive instead of defensive, so they couldn't blame her for the mobs the runners were bringing back.)

Turns out my worry was for nothing. We wiped minutes later (to packs of mobs pulled back by runners!) and the group broke up.