HTTP logging can give a pretty strong indicator of time spent on pages (not perfect, but good with large volumes) as well as navigated routes through the site already.

It would seem to me that the only advantage of this technique is to track mouse movements on the page in a very low resolution, and likely labour intensive way. Low resolution because it only works the first time on each page load, and is going to be tricky to get any meaningful granularity on the data.

I am not particularly concerned about this but my privacy concerns are definitely lower than the general consensus of this site. I only compare to HTTP logging as this is the most hidden/covert way of tracking users.