That's why you rotate everyone, not just those that "volunteer"... This way, you're spreading knowledge to everyone, e.g. if I'm forced to deal with an issue on code you wrote, I'm forced to learn about it.

Of course, I might have to ping you and get you to help me with it, so it's less efficient. Then again, if you leave the company, I have some knowledge about the feature, so... There's tradeoffs for sure.