So does Riot plan on telling you how much mana regen your items give based on base %?

Clarity isn't having every systematical number present. It's being able to understand something based on a quick look. The actual item doesn't show you for some reason, but the store tells you the exact number if you're interested. All 50% mana regen items will function exactly the same as others, if anything, it's easier to find how useful the mana regen is, because it's more consistent at 50% or 100% than 7/5 and 11/5 and 14/5 are.

Furthermore, the mana regen it gives isn't what's important to understand quickly, it's how much you have overall. Things like "this item doesn't give optimal mana regen" isn't something that Clarity needs to show you in game, that's from practice and skill.