On the one hand, it's somewhat irresponsible for Harper to be doing this. It plays into the entire "referendum election" mentality and Stephen Harper is not the best voice to speak for Canada - I think even his supporters would agree to that. Duceppe is a master debater (he he) and would wipe the floor of Harper in a French debate.
That said, this is a friggin' brilliant ploy by Harper. Regardless of whether or not Duceppe accepts, it makes Harper look gutsy. It puts an end to the "in bed with the separatists", "can't stand up for Canada", and "only Liberals can fight separatism" rhetoric, while making Martin look weak. Come the January 9th and 10th debates, Harper can now say "I was willing to speak for Canada when you were not Mr. Martin". The downside is, of course, if he debates and looks bad, but it's likely a beau risque worth taking at this point for Harper, especially since such a debate is unlikely to happen.