Although I tend to avoid YYZ, that is where I have had the best service from the concierge. I do think that the service is very variable, and seems to be less effective outside Canada.
I never really understood why I had a 'concierge'; I just want someone to help me to resolve an issue quickly and efficiently, and to be able to do more than just protect me on a later flight with the same routing. For that the UA staff are generally great.