Why play Spirit Mantle when you can just play Spectral Flight, plus a geist with mantle on it still dies to slagstorm, while a geist with flight on it would not.
spectral flight does still get blocked by lingering souls tokens but i don't get the impression it matters all that much. if you're getting good results from the card, however, i could see either running a split or just doing it because you wanna.
i wanted to try something new, and more often then not i've had my geist die to lilliana or a chump blocker. i haven't gotten the chance to playtest yet. i might try the spectral flight idea though.