My brother went YYZ-LGA on Airmiles w/ wife and son. Paid $400 CDN in taxes. Not all that much better than a discounted ticket (which would give him AP points.)
The taxes you pay on a reward ticket from a FF porgram usually are less than $15 / ticket.
Another bad thing about Airmiles is that they increase the number of miles you need for a ticket way too ofter. I have no idea what their justification is. YYZ-LGA goes up just about every year.