There is a certain amount of vote-splitting between the Liberals, NDP and Greens, and in some ridings this fracturing of like-minded people may result in a win for the Tories. Nevertheless, I don't think we need to worry about the situation, and I don't think we'd be better off if we unified the left.
For one thing, both the Liberals and NDP are venerable old parties with rich histories and an important place in our democracy. Dissenters may point out that the Liberals and NDP have extremely similar platforms. Nonetheless, the NDP acts on the Liberal party as a pressure group to promote progressive policies. Remember the Liberal child-care plan under Paul Martin? That resulted from a deal with Jack Layton, who gave some much-needed support in return for the program. In the early 70s Stephen Lewis was able to use his strength in Ontario to force the government of Bill Davis to bring in rent control and other policies.
Having three parties on the center-left might be a strength. Unlike the monolithic Conservatives, we have a plurality of voices and ideas. The Green Party supports the legalization of marijuana - a tough sell in mainstream politics, but important to mainstream goals such as keeping young people out of the justice system and jails.
The splintering of the left doesn't mean that the Tories will always win. It may be our secret weapon.