You're implying that if nightly builds of a browser with a simplified UA broke a website that the website owners would fix their code, but that is unlikely to happen. Most websites, particularly the sort with bad UA sniffing, have a high cost to change (engineering, QA, making releases) and no incentive ("it broke on the new Chrome, probably a Chrome bug").
Even a relatively flexible company like Google gets UA sniffing wrong for many of its domains. At one point (as an author of Chrome and an employee of Google) I tried to track down the right people to get things fixed and ran into more or less the above problems. (The non-Chrome non-Safari webkit browsers these days must spoof Chrome to not fall into some "other" browser bucket.)
Ah, the pain of working with the development process of websites. I still remember the Hotmail globalStorage fiasco that led to Firefox 13.0.1 putting it back temporarily:
https://bugzilla.mozilla.org/show_bug.cgi?id=736731
The two instances of UA spoofing I know of in Chrome are for large sites -- hotmail and Yahoo mail. My vague memory of the hotmail case is that Microsoft agreed to fix their code but said it'd take months to make the push. (http://neugierig.org/software/chromium/notes/2009/02/user-ag... , http://neugierig.org/software/chromium/notes/2009/02/user-ag... )
Even a relatively flexible company like Google gets UA sniffing wrong for many of its domains. At one point (as an author of Chrome and an employee of Google) I tried to track down the right people to get things fixed and ran into more or less the above problems. (The non-Chrome non-Safari webkit browsers these days must spoof Chrome to not fall into some "other" browser bucket.)