For example, vs Goblin decks, would you rather have Pyrokinesis on their goblins AND your Magus or would you rather Pyrokinesis?
If you're that far behind a goblin player, casting Pyroclasm in that situation would be preferable IMO. Magus is really little more than a blocker against them. Same with elves. Neither deck uses many nonbasics, so Moon effects are not very effective against them.