But if your work gives you restricted oppurtunities to try new technology, or burdens you with outdated tech and boring work, working on a side-project can reignite your enthusiasm in programming, which can never be a bad thing for your career.
Also, theres nothing to say you need to spend huge amounts of time on a side-project, maybe a few hours a month just to try a few new things. Sure, you won't become an expert in those things, but you'll get a feeling of what it's about.
And choose something fun to inspire you, nothing is more mind-numbing that coming home to work on a side-project that actually bores the hell out of you.