Why? Just use the 'relativenumber' option and do 5j.
> the second way is more popular in vim too.
Maybe for moving down 5 lines, but (I hope) not for things like "replace the inside of the brackets with what's in my A register". Otherwise you're really better off using a different editor.
To be sure that the editor isn't in insert mode. Using another slot of short-term memory to keep an editor's mode seems wasteful.
Besides, it might just as well become something you don't need to use memory for at all - just press Esc to make sure. If you're in command mode, nothing happens. If you're in insert mode, you've entered command mode.
Nevertheless it is a limited resource.
> just press Esc to make sure
It's better to press Esc twice. Otherwise there's a risk of terminal emulator interpreting a command as an escape sequence.