That metric would give you a number of bytes which can be used for pages not backed by files, but it won't give you actual memory usage statistics:

It won't count executable pages and memory-mapped file use as "used" memory, so your system might display gigabytes "free" when it's starving, executables getting paused when code pages are paged-in from disk.

It's just less useful than what's displayed now. "Everyone is doing it wrong" is usually a signal that you're missing something.