I'm not suggesting you need to learn the flavor-of-the-month, merely that you don't become so comfortable in your current toolset that you perhaps can't quickly

I'm not suggesting you need to learn the flavor-of-the-month, merely that you don't become so comfortable in your current toolset that you perhaps can't quickly learn something new. For me, it's a matter of assessing whether, in certain instances, language/framework A offers some benefits for the given problem over language/framework B. Too many developers just say "well, I've got this here hammer, and that screw kinda looks like a nail."