Someone did post an itinerary (perhaps a couple years ago), where the Aeroplan taxes and fees (i.e., scamcharges) exceeded the cost of the equivalent purchased WestJet ticket. That would be a definite negative-value case, I think.

I earn miles on MileagePlus, but use CCs and promotions to earn Aeroplan points.

25K + ~$100 for a ~$450-500 economy North American ticket has been my typical redemption method lately. Ostensible, J is a better value, but I'd rather have 2 tickets than 1, so I use my miles in Y.

Although I have not done this yet, it seems to me that it could often be a good deal to redeem 25K on an economy ticket, and go to New York or Chicago or wherever you can get one of those occasional ~$500 deals to Europe (like is currently the case with Istanbul). Then you spend 25K instead of 60K of points, and the cost of the ticket is still less than the Aeroplan scamcharges, and you earn points on your purchased ticket.