For motorsports, I've used, with my 7D, the 100-400, Sigma 120-300 non-OS and OS, as well as the 70-200 2.8 II and 400 5.6L. After using them all, unless you have press credentials, I find my best combo to stick in the backpack is the 70-200 2.8 II and 400mm 5.6L. Fairly light combo, and with teleconverters the 70-200 gives all kinds of options, and when I need the reach, I have the 400mm 5.6L, which I've even used with a non-reporting 1.4TC in good light and had decent results.
What makes me hold onto the 400 5.6L over the zooms is the lightweight, the easy to use built-in hood, as well as the price. I got one for $850, and when combined with the 70-200 I think it's perfect and great bang for the buck. Not to mention I'm always surprised at the results I get at 560mm when using a 1.4TC with the 400mm 5.6L. The 100-400 never took a non-reporting TC as good.
Sure, the Sigma 120-300 OS is probably the best one lens solution, but trying to be up in the stands handling it can turn into a hassle, especially if you have a TC attached to it and you're trying to pan around etc. Not to mention in cases where I'm at road courses walking around, the 70-200, 400 5.6 combo is just easier to handle. It's faster for me to stick the 7D on the 400 than to add a TC to another lens to get the reach.
Just my 2 cents. 
