I heard it's more of a cosmetic thing and doesn't really do anything. Anyone can confirm or deny this? I'd rather not use resource on something that's just cosmetic.

I'd much rather have 50 keys.

Hovering my mouse over the advance *on, I'm met with the statement "Only by activating all hexagons besides the special hexagon will you be able to advance"
I would assume this is the special hexagon it's talking about, so it might not be needed.

And even if it is needed, we won't need it for a very long time. I'm on lv 3 atm, which requires 50 exp, this only requires you to unlock 2-3 hexgaon on the 3rd stage of purple summons each, I would imagine about 4-5 would be needed for next level and maybe 5-6 for the next. There is every chance that we won't even need to unlock ANY stage 4s until lv 7 or so.

And as not all summon have it, we could always unlock the ones where we can pass w/o it. This will last us another level or two.

So overall, except for maybe the final level or two, chances are we won't need to unlock them even if they are required for clearing stages. Thou if that's the case, the amount of unbounded rune needed might be increased, as 3-3-5-5 distribution, for example, will cost more than 4-4-4-4 distribution in terms of unbound rune for the same number of hexagon cleared.