1) Could it played in a lot of archetypes?
2) Is the card a key anchor piece of an archetype?
3) Is the card powerful?
I'm so far on the other side of this argument that it's getting pretty lonely
This is, I think, the crux of the discussion. It's a really different question to ask "would this make my deck?" than "should this make my cube?"
Obviously most decks that play white would rather replace a basic plains with Flagstones of Trokair, but that doesn't mean that card belongs in most cubes.
As well, everyone hopefully understands that the cost of including these cards comes at cutting another card from you Cube not your deck. Cube real estate is hard to break into these days with so many archetypes and generic high powered cards. That means even though each of the MDFC's would be played, does not mean they are auto-includes in a Cube.
We don't play Quench, but Miscalculation is great. We don't play Naturalize, but Wilt is fantastic. I think adding a tapped land option to a spell is even better than cycling (and by a pretty big margin, too). So ya, it's entirely possible for a card to go from bad to good because you add a flexible option to it. Any options we add on to the card that give us more ways to use them have historically been good. Kicker, cycling ...they all give us more than one thing we can do with our cards. There's no greater gap to leap than land to nonland and vice versa, and these MDFCs are going to be awesome. The land option is likely worth about 1 in terms of the acceptable "tax" that it adds to the non-land side ...that's the equivalent of adding 1-mana cycling that can't miss a land when you need to dig for one. So when I look at the Spell//Tapland combo card, if I would cube the spell if it cost 1 mana less, I'm interested in the MDFC. So in this case, I would play a 2-mana clone. So a 3-mana one with a land stapled to it looks great to me. And they change the keepable range of opening hands they're a part of too. Just amazing designs, really.