Scott Adams (of Dilbert fame) wrote about a perspective on learning/training that resonated strongly with me.
"Investors know that diversifying a portfolio is the smart way to go. But does diversification have the same benefits when applied to learning? I think it does, because knowledge acts like a financial asset."
He's looking at the lifetime value of learning a new skill - especially those which may not be an absolute necessity immediately, but will pay off over time.
I agree wholeheartedly. Taking the time to identify and pursue complementary skills has paid off many times over in my career. A key point here - some of the most useful complementary skills I've picked up at first glance have little to do with my day to day work, but in so many ways they make all the difference.