As I told Jan Moren in the comments here, a "grand" coalition sounds about as palatable as breaking two eggs to make an omelet—from the egg shells. But from their perspective, it should beat trying to form a bicameral majority from a motley crew of Komeito, Ozawa’s minions, the Hashimoto Ishin allies, Your party, and what else; or, worse, try to govern without a majority in either house.
The equation changes, though, if either party wins an outright majority in the snap election, since that would give the outright winner a much stronger hand in negotiating a manageable coalition without the other. And the idea that neither party will win a majority* is largely predicated on the notion that Mayor Hashimoto’s Ishin-noKai and its allies will win a significant number of seats. But Hashimoto doesn’t have the candidates yet, who will have to run on their own dime. That’s another good reason for the LDP to push for an early election. In fact, if it were just a matter of reelecting as many incumbents as possible (and not minimizing the eventual gap with the LDP), I’m not sure that the DPJ as a whole is better off playing the waiting game. Of course for a first-term DPJ Diet member, another year in hand is worth much more than slightly improved prospects of remaining in office for four more (minus the one).
* I suspect that the conventional wisdom that gives the LDP 200-220 seats and the DPJ 100 (110?) in a snap election has a single source: a recent LDP, district-by-district survey. Political insiders, journalists, and other members of the opinion-forming classes live much in an echo chamber for matters that they do not have immediately familiarity with. This type of conventional wisdom, on its own, is “brittle” in my book.