Love it! When starting a new release, I asked my team what they would like to focus on. I suggested some of them may want to branch out and work on a component in our system they aren't familiar with. Two of them decided to switch languages, this is how it played out:
Earlier focus >> Chosen focus in new release
C++ >> PERL (job control work)
PERL >> Qt (UI work)
The C++ guy was eager to brush up on his PERL skills, and the PERL guy was really excited about working on the user interface. They both threw themselves into it and had a blast. I think they did their best work. And now they have new skills and experience they are proud of.
So don't be afraid to let people work on things they are passionate about, even if it's not their traditional role. Often it's a win-win for everyone involved.