I think all this talk of Putin not wanting to own Ukrainian territory, but cleverly wanting to dominate territory to avoid the cost misses the point that nobody ever made the history books for extending quiet influence.
No, Putin wants land. He can't take all of Ukraine. But he'd like all of it. And he'll settle for what he can take with this crisis.
And prepare to take more in the next crisis.
We shouldn't cooperate in dismembering Ukraine. There is no reason to assume that their language and ethnic divisions translate into desire for separate states.
That kind of talk just plays into Putin's hands, who would love to go down in the history books as the leader who began the reconquest of the Russian/Soviet empire.
UPDATE: And let me add that for a country with a lack of defensible borders, pushing Russia's flat borders out as far from Moscow is always seen as good. What good does influence do you? Putin thought he had influence with Yanukovich. Then the people got in the way.
No, better to have reliable Russians in charge of the territory. That's the best defense. Screw the influence. That's what you settle for until you can control the land.