I know a lot of people have posted "they don't always enforce it", but if you're taking a trip, is that really something you want to count on? While it doesn't affect me, nor anyone I know, I thought it interesting enough to pass along. I didn't know about it.
In order to take advantage of programs that allow you entry (other than "maybe they won't enforce it"), you need to KNOW to apply. I could easily understand someone who had a DUI years ago, not knowing (or forgetting?) about the rule, and booking a trip that involves Canada. Then they try to go on the trip and are denied.
This came up on Cruise Critic where someone asked if they'd be denied boarding and the answer was "yes".