UA strings are increasingly atrocious, but they're still very useful for targeting platforms and reducing the amount of content sent. Eg, if an iPhone browser comes to my site, the FAQ shouldn't show s40 content by default. And when an s40 user comes, the download section shouldn't link to iTunes (and their browser probably doesn't support JavaScript in a meaningful way)

It's also helpful to put a splash page up when a specific browser release breaks your site, and users need to down or upgrade. (users who tweak their UA may be wrong targeted in this case, but whatever)