Right - if you want to get in for free, you have to be on a Skyteam airline, or US (which I did not know until reading this thread, but then again I haven't flown US in about 3 years).

It's a double-edged sword, however; if you get in for free, so does everyone else, and unless you are a top-tier elite, it is HARDER to get in under a "complimentary" system than a "coupon" system. For example, on AA, I never made it past Gold (the lowest level elite in their system), but I NEVER was unable to upgrade a flight if I wanted to. I was limited by the number of coupons I received (which were, in turn limited by how much I flew), and by my assessment of whether it was worth it to spend $X to buy enough coupons to upgrade a particular flight, but the point is, if I wanted to upgrade any given flight, and I either earned or bought the coupons to do so, I got to upgrade. I never found myself "shut out" because first class was totally full. As a low-level elite on NW, I often find that first class gets filled with higher-status elites. On AA, while I had a limited number of opportunities to upgrade (i.e. I wasn't going to upgrade myself every flight), at least I got to choose when those upgrades occurred. On NW, it is theoretically possible that I get upgraded all the time; in actuality, I find I get upgraded about 25% of the time as a Silver.

In other words, there is a much higher incentive to reach the top elite tier in a NW/CO/DL type arrangement.