They were built to be temporary. I understand keeping the nth and n-1th sandboxes because people still use those. Every time I search for things on meta I get so many trash results from sandboxes it's unbelievable.
In my opinion, the benefits of being able to actually use the smart-search feature on meta outweigh the extremely rare chance that a shining gem of a challenge will be found among the below average, months old challenges.
Delete all but the latest and second to latest sandbox.