Can someone give an example to show why it's a bad idea to have whitespace inside brackets?

Answer 1

It'll work fine with spaces..
But why let those spaces consume some extra byte spaces?

Answer 2

spaces should probably be ignored by the interpreter unless they are inside of the strings or used for indentation, so they should not make any trouble inside of the brackets. and i don't think that saving few bytes in the brackets will make any difference, since they will only affect size of your file and probably not the execution and you waste so many bytes on indentation anyway.