This is what I posted on Loyalty Traveler in response to his Points.com analysis... I'm happy to have anyone tear apart my logic.
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I agree that you need to be selective in your trades. Here is my example of how I feel I’ve been getting a good value trading Airline miles with Points.com.

I’ve been using the Points.com trading board to move out of Continental miles over to Alaska Airlines via trades and am quite happy. Basically I had 50 or 60,000 Continental miles and I’m able to trade 10,000 of my Continental miles and $100 on my side to receive up to 17,000 Alaskan miles. With an exchange ratio favoring me it keeps my trading costs low enough to make it worth it, somewhere under 6/10′s of a cent per mile to move into a more favorable program (to me) – plus my overall number of miles is increasing with each trade… Since July I’ve spent $450 to convert 45,000 Continental miles into 77,000 Alaskan Air miles.

I have to assume on the other end is a willing participant that values the Continental miles much more than I do.