When did we decide that getting accurate serverside log-analysis is definitely more important than saving 100ms of load time?

For people who want an accurate representation of the number of people visiting their sites and the marketeers who would very much like correct data too?

JS Analytics

All client side analytics can be blocked.

Check out Ghostery for Firefox.

Only server-side is reliable.

Not if someone caches your stuff.

Wouldn't it be better to exclude those requests from your stats rather than punishing your users for your choice of log analysis software?

While your idea is good and Apache has the CustomLog directive which could do it, the process is much more complex because of typically customized logging directories (especially where there's more than one client per host).