Sometimes I wonder though.
Lets take for example, +1/+1 counters as a mechanical identity.
I've intentionally included cards in naya, but there's one card in black that would contribute well:
Great in the deck, great on her own, etc. I'm fine including her since even if she doesn't always go in the same deck as
Abzan Falconer or
Volt Charge, she's fine.
But what if that lone black card contributing to the +1/+1 counters theme was
Oona's Blackguard?
It does open up deck space. Technically it also works naturally with a few black cards, like Drana above, or if you connect with a
Carrion Feeder. There might even be one or two Rogues in my list.
But on the other hand, why am I stretching my mana to include the one black card that would actually be good in my deck? Yea sure I could probably add doom blade or whatever, but I've got white and red to get removal from, it's probably not really necessary.
Why am I including Blackguard? It'll most likely go 15th draft after draft, this is what we make fun of amateur cube designers for when they wonder why
Isamaru does the same thing when it's the only white 1 drop in their 1080 card power cube.
I can change that by adding more +1/+1 counter cards to black, but that means taking out cards that play well with my current suite of black archetypes (Not every creature is
kitchen finks after all), and you end up with Gravecrawler players lamenting that she's in the pack instead of
Bloodghast or
Bloodthrone Vampire or whatever it was that you took out that they actually wanted.
Maybe the answer is just run fewer identities in more places, but I don't like that, since a lot of my favorite archetypes are literally incompatible. A prowess deck wants
Dragon Fodder, Pod Deck wants
Mogg War Marshal, and neither deck will bother playing the other because of how the decks operate on a fundamental level. If you restrict which colors a mechanical identity is in (Like restricting Pod to BUG or something, for eg) then you can make sure the colors where Pod is and Prowess isn't have all the pod friendly guys, and vice versa.
If you tear up the rails, every color becomes this contested color where you end up doing the dragon fodder/mogg war marshal debate ten times over, and I feel like you end up either running too many colors to get on theme playables, or you get a "normal" deck running things it really shouldn't be, like a Pod deck with
vampire nighthawk, or a
Wildfire deck with
Sulfuric Vortex