Some very good comments. It is their time and they may spend it how they wish. Some other important points:
1. They may learn something from the apparently pointless task that they may not have otherwise learnt. These new found skills may be useful in other projects.
2. Code they develop may well help other projects solve problems too. Not all code on a closed platform is going to pertain to closed interfaces.
3. Humans actually perform _better_ if they take a break from their regular tasks. Going on holiday is good for you, and working on a side project can be a good diversion too.