The ability to have more than four in your deck is relevant for EDH and casual play. Not to mention the fact that it's a very flavorful and impactful way to introduce the new symbol.
I still don't see the hedron or Kozilek-shape without straining. It looks incredibly neutral to me.
Prowess counted as one of the Khans of Tarkir mechanics, despite going to evergreen immediately.
Why? They issued errata on all modal spells ever when Khans of Tarkir was released, errataed a ton of cards to have Menace when Origins came out, and so on. Not to mention the fact that Dragons of Tarkir slipped in a new type of morph that was retroactively affected by all things that affected morph.
No, they'd need to issue a ruling, or make a minor piece of errata. There would be no functional change. Unless you think that Elemental Resonance doesn't work with twobrid or Phyrexian mana.
"Okay, this means that you can use any color to pay it."
"Cool. I'll tap Rogue's Passage to play Raging Goblin."
"No, you can't, that mana is colorless."
"But you just said that colorless could be any color!"
This is a real conversation that has occurred. If you think that <> would be more confusing than colorless and generic arbitrarily using the same symbol, I have news for you. Being able to tell a new player that this mana is colorless and this different-looking cost can be paid with any type of mana is going to make teaching new players easier, not harder.
...why? It'd be silly to only have the new symbol in mana production. People would be looking to match it up with something, and there'd be nothing to match it up with.