This is a great idea and every company should set time aside for it. Not just a "oh we reserve X percent for issues", but a dedicated, "nothing on the todo list" period.

I worked at a place once where my favorite time was at the end of the year; most people were off on vacation but they needed a skeleton crew in case of issues. This was when we did tons of small bugfixes, improvements and refactorings, and honestly that's the best part of the job IMO. Screw building features and adding customer value, just let me do janitorial jobs :D