I've heard all the excuses, and none of them have won my heart over to the monarchist camp.
1. "It's our heritage" - no, it's British heritage
2. "It's what distinguishes us from Americans" - no, if that's what you think really separates us from them, then you have a piss-poor idea of what a Canadian really is. Besides, if we insist on distinguishing ourselves from Americans based on meaningless trivialities, we might as well do something totally useful, like converting entirely to the metric system, etc.
3. "Keeping them saves a few bucks. The Queen is free" This is the worst one. This is the part of the Canadian psyche I don't like: when we think of money before substance. The price tag doesn't matter, what matters is the ability to unleash ourselves from years of cultural servitude. Even if you think it's benign and harmless, I have to stare at the Queen's mug everyday on my twenty dollar bills and coins reminding myself that we are still not entirely masters of our own house.