Having a lot of parties in Parliament is not the problem. Having a lot of parties splitting the vote in every single fucking Riding is the problem. The SoCreds/Credistes, for example, were never a national party because they never really competed outside of one of the four Canadian regions. Reform was not a national party until the very end, right before it ate the PCs. The Greens are.
I have read many Green apologists claim they take votes from parties that are not left-wing. Pretty much every analysis of this phenomena I have ever seen is pure motivated reasoning. Minuscule sample sizes frequently appear. What actually seems to happen when political campaign types strongly analyze the data is that much of their vote comes from the left--wing party, and every-goddamn-body-else would have been a non-voter because they're incredibly angry at the political system.
Think about it this way for two minutes: the Tories stand for no gay marriage, lower taxes, more weapons for the Armed Forces, strengthened links to the UK, ending the Kyoto treaty, siding with businesses whenever environmental activists complain on principle, strong support of Israel, near-slavish subjugation to the US in every important foreign policy arena, etc. The Greens strongly oppose all those positions. Yet many analyses claim that half of Green voters care about these issues so little that their second choice is the Conservative Party.
What do you think happens more often: a fairly far-left activist whose so angry at NDP/Grit impurity on the issues (not their ineffectiveness in fighting for said issues as minority parties in a majority-Tory parliament), vents by claiming the Tories are his second choice to some idiot who doesn't understand human psychology; or that there actually exists large class of people whose first choice argues that a) LGBTQetc. rights are the moral issue of our time, c) the military should be shrunk to the minimum conceivable, c) Global Warming is a man-made disaster and EVERYTHING should immediately change to reduce it's impact, etc. but has as a second choice Stephen fucking Harper.
As for your definition of Canadian left, in Canadian terms you are right. The Liberals define the center ground. OTOH in terms of English-speakers on the North American continent the Liberals are really truly fucking Liberal. Prior to the PC-Reform merger they could win because the moderates right-wing party had it's vote split. They even managed to turn their votes being split into an advantage, because they could use the NDP for cover to move left when they wanted to create Medicare and still play the moderate.