If someone has a client that can't render a standard page, then that's their problem and we should have left them to it - eventually they would have complained to the relevant person and their browser would become closer to the standard.
Are you new here? You may not remember the days when this mess all started. IE was king and you *had* to work around it. You couldn't just let it be "their problem" and "left them to it". That's so "counter-productive, I can't even begin to explain it". These customers (sheep running IE) would come to *you* in droves asking why they couldn't view your website. And your response was going to be "because IE doesn't display my standards-compliant page"? Wow...no...that doesn't work.
Nowadays, things are clearly different. Which is great. But to suggest developers should have never used the user-agent tag to distinguish browser differences is ludicrous.