SQL*Net message to client wait isn’t really what it’s thought to be

In a recent Oracle Forum thread a question came up how to use SQL*Net message to client wait events for measuring network latency between server and client. The answer is that you can’t use it for network latency measurements at all, due to how TCP stack works and how Oracle uses it.
I’ll paste my answer here too, for people who don’t follow Oracle Forums:
As I wrote in that reply, “SQL*Net message to client” does NOT measure network latency!