Sorry, I was more vague on that initially than I should've been. I've answered this elsewhere, but: what I was referring to was delivering value on behalf of someone else vs. using a given problem as an opportunity to learn something new. Delivering value (e.g. adding a new feature to your product, fixing a bug in a shipping product, identifying and attacking a market opportunity) shouldn't be seen as an excuse to learn a new technology unless you have absolutely no pressure to ship that work on any sort of schedule at any presumed level of acceptable quality.

I'm the same way: I need a real problem to work whether I'm applying skills I know or skills I want to learn. I just don't ever promise anyone (except myself) anything when I'm applying technologies I don't yet know to the problem.