I think if you work on the web you need to know HTML, CSS and JavaScript to some extent. It's great to specialize and to get really good at your core skills but just to be able to work with the rest of your team well it is worth becoming reasonably well-rounded. HTML, CSS and JavaScript are the bit the browser sees, and you can have the most amazing backend but it will be let down if the frontend is clunky.
I'd also say that often people who have managed to avoid the frontend for a while are pretty surprised when they see what is now possible. I love presenting on CSS to groups of backend folk, as quite often they are blown away by the stuff we can now do on the frontend, and get quite excited about it.