Even for true side projects, the solution is to make sure your project schedule doesn't depend on everyone working a strict 40 hours. Having time set aside to do some OSS hacking every week is actually likely to lead to less time wasted with other, likely less productive activities.
You also never know when one of those completely unrelated side projects will come in handy.
You also never know when one of those completely unrelated side projects will come in handy.