Why take Clarity when you can build mana regen? Again your argument is why take a summoner spell when you have runes and items and that's exactly my argument which you are borrowing and that doesn't make a good case for you
The entire point was to prove that your argument was silly. Yes, you can take mana regen runes instead of new clarity, but nobody would do that.
You can also take Ghost right now instead of movespeed quints. It's the same argument, but apparently you're not looking very deep into it, or even shallowly for that matter.
You're not forced to buy anything as any champion, but some things help more than others. For instance, people take ignite to usually supplement their innate lack of damage, and people take barrier and heal to supplement their lack of innate defensive stats. Therefore, people should be able to take clarity reliably to supplement their innate lack of mana regeneration. People take those spells so they don't have to build things that would skewer their build path. Malphite is a great example of this. Also, mages with very high mana costs.
I am not saying Mana-hungry champions should never buy Magic regeneration. I am saying that they should have the option to have more mana regen which would make it so that they aren't forced to buy a mana regen item as their first buy or face the consequences. The mana regen is worse than that of a chalice. It would supplement their early-game. For some reason, you seem to be thinking that the mana regen granted is going to be upwards of 10 mana/second, which is not true. If I must, to sate you, the mana regen passive will only be up when Clarity is on cooldown.
The entire point of this thread is to try and make it viable. The entire point of clarity is to give mana, thus making mana-hungry champions less mana-hungry. That is the purpose of the spell, is it not? It gives mana. What did you think it did?
It definitely doesn't take skill to land your abilities as Malphite/Liss/Ziggs/Orianna/Lux, and it's not very hard to do so. Again, it's a summoner spell. I highly doubt it would be an instant-grab on midlaners over barrier/heal/ignite, but would become a contender and would also help warrant the rise of mana-hungry top laners, offering more viable picks to competitive play that are heavily gated by their early/mid-game mana issues.