I can only speak from my personal experience. My husband (American) had a primary residence in the US (i.e. property he owned and had lived in) which we listed as his residential address - since that's what it technically was. However for several years, he had been spending the majority of his time staying in Canada as a tourist with me (he's a consultant and has flexibility in location and travel) and didn't have access to his home mail consistently. So we listed my home address in Toronto as his mailing address. We had supporting evidence in the application that demonstrated the US address was in fact his home and his primary address while in the US. In our relationship letter (or whatever the heck that thing is called), we were transparent in saying that my husband had been spending a great deal of time (most of his time) in Canada staying with me in my home. CIC was perfectly fine with this and all of his mail was sent to my address.
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That was exactly our situation. My husband maintains a residence in the US where his mail comes in but he is staying with me in Canada as a visitor. We put his American address as residential address and my Canadian address as his mailing address (so that we get the mail faster, without his brother having to forward it to my address). CIC returned the application. Following advice from this board's members, we filed a new application with my Canadian address as his residential and mailing address (with a note that he's staying here as a visitor). It's been almost 2 months and they haven't returned it. I'm expecting the AOR any day now.
I honestly believe that it all depends who processes your application. Our first version perfectly made sense, with plenty of explanations, in several places re: his address. It didn't work for that particular officer. I know that some people (like scylla, who provided plenty of advice when I was filling out our application and had it subsequently returned) had their application go through smoothly. I wasn't this lucky and got put behind a few weeks. I'm just sharing my experience and having read multiple threads about this issue, CIC is pretty inconsistent when it comes to the address issue, which results in a lot of frustration because there's no clear guidelines.