What would the harm be? It already has a 4 minute long cooldown, a whole 240 seconds, and offers nothing to its user besides a 50% Mana refresh. So why not?
we'd get more oppresive toplaners who already lane bully 90% of their lane opponents with no trouble and who only have to back for mana or if the opponent is 
or 
due to them healing.
Having clarity locked behind unsealed spellbook makes it so that things like 
having both electrocute damage and shittons of mana aren't a thing or 
would only have to back for items so she'd never leave lane... and considering her already famous 1v1 power being (pretty much) the best in the game it's no wonder why clarity isn't coming back.
+ Clarity = one of her 3 main counterplays are gone since now you can't just "outlast" her in lane to outscale her as she can always stay more than you.
and 
instantly become impossible for any toplaner, now, to counter because as of right now the only toplaners who can "counter" these three are all able to heal shittons of health, those three being 
but yorick doesn't even have the teamfight power of the other two so he's only really a "counter" for the lane phase as he can still be relevant afterwards but not to the same degree that Mordekaiser or Aatrox could.
is now unstoppable toplane against any melees as their one chance is to, yet again, simply wait for her to run out of mana so they can catch up in farm... but if she can have clarity and just get 50% of her mana back anytime then toplaners go from not farming much for 5 minutes to not farming much for 10 minutes... in a meta where lategame doesn't exist and the only chance for most toplaners to be relevant is to get WAY ahead of their lane opponent, this spells death for all melee toplaners besides like zed or talon.
This is just toplane that i'm talking about though...
Toplane would get absolutely fucked entirely and upside down just all the way.
Midlane however would see a major increase in the diversity of champs that go there...
Mage supports(who used to be midlaners anyways) would become more relevant and could start actually coming back to midlane as they'd no longer run out of mana as quickly. They'd still end up being out-paced by the current midlane meta as the current midlane mages, assassins, and divers would just abuse clarity or keep abusing ignite to keep up a better pace than the mage supports could realistically keep up with.
But midlane would see roughly 4ish characters go back so [slayer-pantheon-thumbs]
