Nations surrounding Canada (not many to begin with) don't often run into natural disasters on a scale the communities cannot handle on their own. That kinda eliminates the HADR role for a Canadian Mistral.
While this is true, mission like these (Haiti as an example) are widely supported by the Cdn. public and politicians alike. It feeds into this sort of narcissistic attitude Canadians have; we like to show off to the world how giving we are and how, unlike our neighbors to the south, we primarily use our military to help / rescue people and not conquer them (not my views - just trying to describe Cdn attitudes on these matters).
If the gov can sell it to public that we need them for disaster response then go ahead. While the debate in Canada seems to resurface almost every few months now I still don't see it happening, but who knows! Just don't trim the budget for the surface combatants! As for manpower, the RCN would actually quite easily be able to man them with the recent decommissioning of 4 large surface vessels in our fleet so that wouldn't be a big problem.
Personally, I'm intrigued by the flexibility offered by vessels such as the Mistral. I think I even remember reading on here that some countries have been exploring the possibility of LHD being utilized in an ASW capacity - loaded with ASW helo's. I think with the coming generation of UUV, USV, and UAV's - vessels like Mistral will be highly relevant and sought after by modern navies.