Harper refusing to show up at debates initially looks childish and petulant. Especially from someone like Harper. I hate his policies, but he's a decent debater and certainly a shrewd campaigner, which makes him all the more dangerous.
But as I said, his original refusal to debate seemed childish. When Mulcair first said he wouldn't debate at any forum that Harper didn't attend, I felt like he was lowering himself to Harper's level, but a friend of mine put it in a different light for me. Mulcair's refusal to show up at a debate that Harper refuses to attend makes him look principled. If you think about it, Pierre Trudeau would never have attended a debate that Joe Clark didn't attend. Mulroney would never have come to a debate that Turner refused to show up at. So, that's a new way to look at it. Mulcair is taking a principled stance against the petulant child strategy of Harper, which now makes Harper seem obstructionist in addition to petulant.
Without too much work, Trudeau and Mulcair can both make Harper look like an idiot over this.