There are use cases for setting a cell value.  One such use case is for (un)pickling recursive closures (see heroic workaround here: https://github.com/cloudpipe/cloudpickle/pull/90/files#diff-d2a3618afedd4e124c532151eedbae09R74 ).  Other use cases may include tinkering around and general education value.

There also doesn't seem to be, AFAICS, any counter-indication to being able to do so.  It's already possible in C using PyCell_Set(), which is a public API.  It just lacks an API in Python land.  For example `cell_contents` could become a read/write property...