dfb,
diddagirl is right BUT there's still a chance with you being denied entry into Canada with a passport alone. My wife had a work permit while going through PR and there was no problem with her getting back into Canada after a trip back to the UK. It's not needed but if you have a work permit it gives you reason to be here not just to visit and the IO's won't wonder if you're going to leave or not after the 6 months are up.
Yes, as diddagirl said above, the WP with make it easier to re-enter with the passport from a visa exempt country. Everyone needs to show a reason they are here even if it's just a holiday. The WP will smooth the way.