The problem isn't spells requiring two colors. The problem is spells requiring all 3. Let us go over a list of the good cards in the bant deck real quick. The ones I am worried about taking over a game.
3
rhox charger4
rhox war monk2
finest hour2
jhessian infiltrator1
jenara, asura of war1
rafiq of the many2
sublime archangel3 things I notice immediately.
1. 10 of those 15 cards require all 3 colors.
2. All those cards need you to have a ton of creatures on the field to be effect a lot of games. There isn't really an option to not overextend here.
3. This deck only really attacks with one creature.
Rhox Charger is your only reliable way to force damage through without flying, and versus flying decks that's not a guarantee either.
While there are ways to get around these issues, this is still the only deck in game where I actually lose due to color screw on a very very regular. Not mana screw but color screw. The other 3 color decks don't have that issue. I don't get color screwed in dragons, and I don't get color screwed in slivers. And my definition of color screw isn't missing a color for a while and playing around it. My definition of color screw is I mulliganed twice and I only got to play 2 cards all game because that third color I need never showed up. With 26 lands and both cyclers in the deck, this is still a very real issue. Now, once you get past that issue, I have to deal with my creatures getting chump blocked. All day long. Because this is an exalted deck. And exalted has that problem. So everything with evasion immediately merits a look. Because it has evasion. You know what the trouble is? A lot of the exalted things don't have evasion. And that's most of your early, not requiring 3 colors stuff. Throw in that monster destruction and field wipes magically exist everywhere, and this really has some problems. I'm sorry if people don't like to hear this being beaten to death, but when I am confident I can go up against a deck best 2 out of 3 with 15 out of 19 decks in the game, and not have to worry very much, in 1v1, the deck has issues that can't be glossed over by how well it does when it's issues don't flair up.
Edit: And I don't know what ^^ is playing but 4
moroii is enough for that matchup to be considered in Dimir's favor when comnbined with the discard, the stall, the copying stuff, the bounce, the 6 mana bombs, and tons of other things.