Francophones already comfortable with the PQ's rhetoric are unlikely to switch unless they're given a reason to. The strategy doesn't leave the Liberals with much identity of their own. True, the party's federalist orientation is one distinguishing characteristic. Another is its faith in the private sector and its belief in smaller government. There's real risk in trying to co-opt terms like nation and state.
Words do have meanings.
If the Liberals concede that Quebec forms a nation, it might not take much of a leap for voters to conclude that a nation deserves its own country. By grabbing on to the PQ's nation concept, the Liberals are taking away a potentially useful weapon. They won't be able to critique the PQ's use of the term. They won't be able to argue that Quebec, with its increasingly diverse ethnic composition, has no real basis for nationhood. This is the same kind of mistake Liberal leader Jean Charest made when he promised to seek a referendum on constitutional change.