With talk of the potential acquisition of Ryan Smyth being thrown around, you have to stop and consider the salary cap implications that a deal like that would entail. With his contract in the range of 6 million dollars, and no guarantees how the cap number will change in the coming years, you'd be seriously impacting your potential to re-sign upcoming UFAs like Tanguay, Koivu, Kovalev and Komisarek, as well as RFAs Carey Price and Jaroslav Halak in the following year.
If a team was in a situation where the addition of one piece would put them over the top in their quest for a Cup then I'd likely consider the move and let the future take care of itself. Conversely, when you're in a much more ambiguous situation like the Canadiens find themselves this year, you have to weigh the relative pluses and minuses of a deal of this magnitude. In the "new" NHL salary cap room is as powerful a tool as the talent on your roster, and handcuffing yourself behind a bad contract (no matter how good the player) is just not a good way to go. If I were the Canadiens I'd hold on to my prospects and cap room rather than crippling the team with a trade for somebody like Smyth or Lecavalier and wait to see what the offseason will bring. With plenty of cap room available when all of their UFA contracts expire this summer, the Habs will be in the position to offer deals to the best talent available (UFAs on their own team or others, as well as through RFA offer sheets). While it may not seem to be the most appealing way to go in this season of high expectations, the team will ultimately be better off if they choose to avoid the deadline "quick fix" and instead take the long-term view.