If all sites had proper favicons for branding we could start dropping site name from page titles and focus just on the content.
ETA: To expand, I just opened the top 12 stories on HN right this moment in new tabs. I recognized the NYT logo in one favicon and the Apple logo in another. Of the other eight that did have favicons, those little icons told me absolutely nothing useful. A few of those sites did indicate their names in the titles, however.
And why would I have to visit billions of sites? The issue is that billions of sites exist, and a random little picture tells me nothing about what site it's from unless I already have it memorized.
How useful to you is an unfamiliar favicon, or a row of tabs with an assortment of unfamiliar favicons?
That it actually quite useful when I flick back to a random browser window after being distracted... ("Ahhh, that's my timewasting window!")
Having only icons would be such an accessibility disaster.
2) Chances are also that I might want to know which site something came from at a glance.
For bookmarks that you already have: right click, edit.
Personally, Google has replaced my bookmarking behavior. I don't bookmark pages any more - when I need to search for them, I just google for the appropriate keywords. In fact, this works so well, it's rare that I even need to add a "site:" to give a hint as to which site to look for it.
In reality it should work fine because searches also include the URL so you can easily see the source (same idea as used by HN items). Titles are easier to fake so can't be entirely trusted anyway.