I think that purge as a mechanic is fine, it’s just how nicely it slots into some things a little too well, both in terms of how the spec does damage (Shadow) and mana cost (also Shadow).
Eating up a bunch of mana and/or globals to offensively pressure somebody that would’ve been covered by Hots is fine as long as it heavily dents the mana bar, and/or disrupts their ability to do damage.