I disagree. I think that on the contrary, the Tories played this one brilliantly. By initially threatening the PM's boycott of the debate, they laid the bait which was taken by the NDP and the Bloc. Jack Layton has been on the defensive ever since, and not just with the media, but within his own party. It had the so-called 'progressives' going at each other for a good 48 hours, and swayed them off-message nicely.
For Harper, the decision to oppose May's inclusion had little cost - Green supporters aren't likely to be swing voters the Conservatives could hope to win anyway. By the same token, he doesn't stand to lose many votes by her inclusion in the debate. Both by initially excluding May, and then by including her, the Tories have simply encouraged greater and deeper divisions in the 'progressive' vote. I suspect a brilliant mind may be behind this devious plan, and I love it.