Yeah because the Liberals propping up Harper would be all peaches and cream for them. The whole point of Iggy's and the Libs' campaign was for Canadians to vote for them to get Harper out. That's why they kept saying "don't vote for the NDP if you want Harper out", etc.
So then, if they prop up Harper, all of the voters who voted for the Liberals to try to keep Harper out would be lost to the NDP... pretty much they would lose their iron hold on Ontario/Toronto.