On what browsers or user agents that channel URL is actually used, and what for?
I have no intention of having my site to work on Internet Explorer <= 8 (it is an HTML5 <canvas> game, and I am serving everything else as "application/xhtml+xml").
So, if channel is only useful on that old crap, I can gladly get rid of it...
Related (possibly): Channel URL Facebook
Because the social plugin is cross domain call, it needs a way to communicate. The wrokaround is to include a hidden iframe in the page for that. But, with this workaround, that iframe is loaded every time when page loads and will double the traffic reported. This is why channel url was done. What it does, it load the fb js in that page, and from that moment on, the js is available on your domain.
It will improve your loading times (cache) and will fix the reporting issue (you will see in reports channel page reported separately). But is not necessary for any html5 capable browser.
So, if you are using only HTML5 capable browsers, you are safe to ignore that. I am not sure about ie9, I will try to test it with my app by removing channel url and let you know.
Edit: By removing the channel URL from my app, I start getting double traffic reports from IE9. I think that is a good idea to keep the file there, is is just a simple html file with a single line. Better to be safe than sorry.