This is a tough call. Everyone says that HTML 5 will replace Silverlight, but HTML 5 and Silverlight aren't 100% overlapping technologies. There is a LOT that Silverlight can do that absolutely, positively cannot be accomplished via HTML 5 (and still achieve the same performance / user experience).
I wrote and maintain a very thick web-delivered imaging application using SL. There's no way we could have implemented the features and functionality through HTML 5 + AJAX and still achieved the same level of performance and usability... not to mention the nightmarish amount of Javascript that it would have taken to even come close.
That said, it does seem like Microsoft is abandoning it. And despite the project I just described, I'm not against Microsoft abandoning it. I'm sick and tired of idiot managers trying to shoehorn thick-client applications into web scenarios (usually because they don't understand the differences between the two). If you need thick-client performance and functionality, then write a thick-client application with WinForms, WPF, etc.
tldr; No, I wouldn't bother unless it's a requirement for your current job or your dream job. The benefit is that a lot of what you learn will transfer to thick-client development (WPF), so it's not a total loss.