From the CIC website:
Your PR card can be used to show that you have permanent resident status in Canada. If you travel outside Canada, you will need to show your card and your passport when you come back on a commercial vehicle, like an airplane, boat, train or bus.
PRs traveling outside Canada who do not have a valid PR card, or who are not carrying it, need to apply for a permanent resident travel document before returning to Canada by commercial vehicle.
When my son was in the US for Xmas with his mom, his PR card expired. There was no way to get it renewed that fast, nor get a PRTD. I talked to a couple people at CIC and they said he would likely get refused entry without the card or a PRTD. I ended up having to reroute him to Buffalo and go get him from there, as driving across in my own vehicle required alternative options.
Which is why you don't mention it to the gate agents when you board the plane. There is a reason they won't let you board, because they would be on the hook for flying you back should you be refused entry without proper travel documents.