Poor Marc Garneau, he wanted a one on one debate with Justin Trudeau and Trudeau turned him down. That was a smart move on Trudeau's part, dumb one on Garneau's.
While it's true many people would like to know where Trudeau stands on a variety of issues, there is no reason for him to get involved in a one-on-one debate. Right now he is the front-runner and he has the most to lose with such a format.
If Trudeau is ahead what point is there in accepting such a challenge? It would put him at risk of making a mistake or in this case, having Garneau prove to the public and media (those that don't already suspect it) that Trudeau is "policy light." There is simply no upside for Trudeau in accepting such a challenge.
In addition, if he had accepted the Garneau challenge, Trudeau would have indicated to the other challengers, delegates and registered leadership voters that he was concerned about Garneau's momentum and that he needed the debate to knock Garneau down a peg. Accepting would have raised Garneau to equal status as a front-runner, something he is clearly not at this time.
Trudeau also had to look beyond the debate as well. Losing a debate to Garneau would have given the "anyone but Trudeau" folks, as well as the supporters of other candidates, someone to rally around to stop the Trudeau juggernaut. It certainly wasn't in Trudeau's interest to risk providing that opportunity.
As for Marc Garneau, outside of the temporary blip in media interest that his press release caused, did he seriously think Trudeau would accept? Such a move also signals that Garneau just might be getting a bit desperate that he hasn't closed the gap on Trudeau, either in supporter numbers or dollars raised.
Garneau's challenge to a one-on-one with Trudeau also brings into question whether or not Garneau is a team player. How does he think the other leadership challengers will view his wanting to bypass them? If he is so willing to insult them (and their supporters) by wanting to bypass them at this stage of the leadership race, does he still expect their support later on? Win or lose he still has to work with them in the future.
Certainly Garneau's challenge provided a few minutes of interest in what so far has been a pretty dull leadership race, but it went nowhere. Did the challenge help Garneau move ahead in the contest? I doubt it; instead he might have alienated more supporters than he gained.