I have shot only video for many years and I am not saying it is right or wrong but almost any professional video camera system with interchangeable lenses does not have an auto focus system of any kind. I have used them to shoot sports, weddings, commercials, etc with no focus problems. The difference is they have a much higher resolution viewfinders that assist in getting an accurate focus. But for the moment lets talk of the professional cameras that do include an auto focus, they have some trade offs. For one they typically have very small sensors so the DOF is never very shallow so if it misses focus it is much less noticeable. Also the focusing mechanism is not extremely fast (compared to an SLR), so things in the shot roll in and out of focus instead of violently snapping around, this is much more pleasant to the eye. The lenses are also designed for video, they are extremely quiet.
Secondly lets look at the physical hardware requirements for a true auto-focus system on a HDSLR, many of which would cause compromises to the still picture quality and functionality. Look at Sony's system that requires a semi-transparent mirror that results in a loss of light to the sensor and requires a digital viewfinder. The other issue as I mentioned above is still picture lenses are not designed for video, they are extremely noisy which is a nightmare in post, not to mention the focus mechanism and algorithms are designed to achieve focus as quickly as possible, and it is not very pleasing to the eye.
Now I am not against auto-focus, there are times it could be helpful (boom or steadycam shots), but even then from my experience they cause more missed shots than good ones. Everything requires some trade offs, I think a lot more people would be upset if the still picture quality/functionality/speed was compromised to please the people that want/need a full time auto focus system. At the end of the day it is a still camera with video functions.