I really dont think anyone is going to come on board a campaign for being promised to be a certain critic or Deputy Leader. If anything, calculation is going to lead to the conclusion that the candidate you are linking to only has an X% chance to win... and if he doesn't, you've set yourself back.
As counter-intuitive as it may seem given the response here, I think Dewar is doing it out of the idea it is good optics. Whether it turns out to be the case remains to be seen. Not around here, initially at least.