First the moon phobos is held in place by the cabal so destroying the flayers would cause it to hurtle out of control. Battling Crota and atheon was not a test of strength. Still nice argument
English
-
I don't see how Phobos is a problem to the two. They honestly wouldn't care if it dropped onto a planet inhabited purely by evil alien armies. Hell, Vegeta would probably throw Phobos at Mars just to rack up kills.