I've found with the ETC on my 2004 D2 that there is a bit of spin but then the ETC kicks in and the grounded wheel gets the traction. In fact, I believe that there has to be some spin to kick ETC into action. If the D2 wheel in that photograph was just constantly spinning then this is contrary to my experience with ETC.
I'm no expert, and I'm sure that many on this board have significantly more experience than myself, but I can tell you that I've been impressed by the capability of the D2 with ETC and I've also found its limits.
If you have at least 3 wheels on the ground with some sort of traction, ETC + CDL will pull you through. If you find yourself in a 2-wheel type scenario where you're cross-axled with minimal traction on one front and one rear wheel, you'll need to rely on momentum or you'll be stuck. I enjoy some challenging rocky trails, and find myself in this situation more than I'd like to admit, which is why I chose to spring for lockers.
I'd rather be able to crawl out through in a controlled manner than need to rely on speed to bounce me through.