whether or not it's worth your time depends on why you want to learn and to what extent you want to be proficient at it.
here are the best reasons why i think you should delve into the front-end:
- you want to be able to bootstrap reasonably well a site from front to back - you don't want to be completely reliant on a front-end person - you want to become a better back-end person by learning the needs of the front-end - you want to fully appreciate the value a front-end person brings to the table - you want to have more control over how users directly interact with and perceive your product
if you agree with any of those reasons above, you should certainly consider delving into the front-end. however, let me get to my second point.
if you're going to jump in, i recommend you afford yourself the time to really get deep into the subject matter. why? because we've seen a tremendous diversity of front-end design patterns/methodologies sprout up in the last decade; all of which can have a sizeable effect on how the back-end is designed.