A Question about Cleanse

So I was never quite sure but just confirmed that cleanse does remove debuffs, unlike Mercurial Scimitar. Those debuffs include exhaust, ignite and other such things.

So my question is, since exhaust tends to screw assassins over really hard why not take cleanse over ignite for Champions like Katarina? I know that it's less all in early on but shouldn't it help more overall?

I play support a lot and exhausting the enemy assassin when they go in to kill your ADC can be game changing as well as just hitting the CC. Cleanse in theory should allow them to assasinate better but you only really see it occasionally on immobile midlane mages.