Yep, I must say that I was initially irritated when I read this. Because a lot of websites do this stupid "Check to see if your browser is IE or Netscape" and then dump you to a "We're sorry, you have a stooopid browser" page if not. The really irritating bit about this is that once past that initial check page, it works fine no matter what browser you're using...

So yeah, upon reflection I can think of reasons for OS detect. But more often I have encountered bad reasons for the OS detect.