This thread has a lot of branches, so I am just picking one for this general reply:

I recall reading somewhere, years ago, that some OSes couldn't be relied upon to release unfreed memory when a process terminated. In those contexts, fastidious freeing would be important even in short-lived processes.

In my own C code I tend to free everything so that I get a clean trace from Valgrind and don't risk masking legitimate bugs, but I typically write long-running daemons.