It's a false sense of security though. It's like putting a big badge on your website saying it 'validates' - even if it's totally broken in real world browsers.

In my experience, the more common bugs are integration bugs, scaling bugs, weird memory usage issues, not to mention interfacing to external systems - for example some weird browser comes along and does idiotic requests you'd never expect a sane person to do.

Those things are very hard to test for, and don't really produce a good ROI as abstractbill says... Best just to log things, graph things where possible, spot weirdness, investigate and fix.