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Making dual pillars for ALL the different 2 element combinations will yield a LOT of dual pillars. These are basically more powerful versions of pillars. We already HAVE pillars; why would we want more powerful versions of them just to unbalance mono and trio decks? It would just create more confusion.
First, they are not pillars, but permanents, so only 6 in the deck.
Second, they have a price, and it's not a thing to understimate... You need to pay to play them, so they need normal pillars to be played and are slower (even if more "powerfull" then them).
And, anyway, they are just different... In a mono deck, they are pointless (a lot weaker then a normal pillar).
They are like what quantum pillars are for raimbows... Quantum pillars are a lot more powerfull then normal pillars (couse they generate 3 quantums per turn), but they are suited for raimbow decks.
That permanents are just a tool to make real dual (not just mono with different mark) and trio decks, so they are not just "normal pillars more powerfull", they are a different thing.