Several browsers, including many versions of Firefox, had a bug relating to the status bar. The "waiting for ..." would only update after the first byte had been received from the next file. Thus, a slow resource would leave the status bar blaming www.google-analytics.com.

The bug has since been fixed in Firefox -- I don't have the url handy though.

IMO this is the main benefit of the async snippet -- random server problems won't lead inaccurate metrics.

(This is most visible with adwords campaigns -- you can see the number of clicks you paid for, but the number of visits they lead to is less. Typically due to a combination of high bounce rate plus slow loading pages.)