I have done maybe 20 or so interviews in the last 2 years and this is the #1 thing I look for. However, I have still hired people who do not display a talent for continuous rapid learning, that is to say, they will only learn on company time and they take a long time (reasonably) to learn anything new. The flip side is that these people "work with what they know" and you can rely on them for a steady work pace and consistent quality. Much like so many development tools, people come in lots of shapes and sizes that benefit your organization differently.