So you are going to take the time to correct hundreds or sometimes thousands of warnings in some open source program you need to use at your job?

No one said that... -Werror is good when you are developing, not when you are compiling someone else's code.

Every good programmer I met enables as many warnings as possible and strives for zero warnings. This is just professional practice like keeping your code in a version control system.

gcc/clang warns about this particular problem without any extra command-line options. You need to go out of your way to stop it complaining.