Feeding is only illegal in western Manitoba, it says in the guide its not "encouraged" in the rest of the province. I believe its to slow the spread of CWD.
I read a piece by Cody Robbins in Big Buck magazine awhile back where he was defending feeding and baiting deer, I thought it was really good, but the one thing that I really got out of it was the off-season feeding (after hunting season until spring) was helping to keep some of the deer alive. The bucks are in rough shape after the rut and are heading into winter, this gives them the boost they need to make it through the winter (and be there next hunting season.......).
I actually am considering doing some off season feeding, start in December and keep doing it until spring, after that the deer can feed anywhere they like, likely far from where I'm hunting in fall. But it might depend on how a winter goes, if its a "soft winter" then they don't need the help.
(probably get my hand slapped by Andre for this post (where is a "whistler" icon when you need it))