... in particular the argument that "you should not enable warnings until you really understand them" strikes me as a bit absurd.

I used to agree, until I tried to get some not-terribly-complex C code to compile on several different platforms with several different compilers. There are some awfully arcane and unspecified corners of the C language, and there are some awful behaviors in vendor compilers.

Where possible, it's nice to get C code to compile without warnings, but there are a lot of cases where you have to make a lot of tradeoffs. Complex casting macros may be one of them.