Despite unsubstantiated assumptions, if Duceppe is offering more than just bluster, then the Liberals really have no choice but to vote against the government. In a scenario where the NDP and Bloc reject the throne speech, it is tactically improbable that the Liberals could prop up Harper, without negative consequence.
Prior to the by-elections, Dion had ramped up the rhetoric and was positioning the Liberals as anything but conciliatory when it came to the core issues, that could trigger an election. It seems logical, that the Conservatives strong showing in the by-elections, will translate into more rigid vocabulary in the throne speech, which leaves even less room for compromise. Within that environment, election readiness and organizational problems take a back seat to political necessity. Just imagine the howls from Layton, the new-found credibility of Duceppe, if the Liberals are left in supporting the government. The appearance of a toothless tiger, scared to face the people, more consumed with survival than defence of policy is real.
The Liberals will vote with the other parties, if that actually happens (Duceppe has been known to change his mind quickly), because they can't afford to look irrelevant, emboldening the other opposition parties. Abstaining from the vote is a strategic non-starter, because the optics are still the same, as a matter of fact the term "gutless" receives context. I don't really see a choice.
Despite problems, it is hard to imagine things drastically improving for the Liberals in the next few months, the relative landscape not as bad as the last week assumes. Heading into an election, tied in the polls, isn't exactly disaster. Whether illusion or not, the prospect of success is at least plausible, particularly when you consider people's hesitation with Harper. You could go so far as too argue that an election provides Dion the opportunity to "connect" with Canadians, which he desperately needs to do. An election also deals with the "what do the Liberals stand for?" criticism, because we would finally see some fleshy policy. Ideal conditions, no, but there is enough there to justify bringing the government down, especially when the alternative is isolation and the visual of collusion with the government.