Yes, I get that inverting signs gives you symmetric values. But how about mixing signs? If you mix positive and negative nines, you may reach some numbers unreacheable any other way. E.g., suppose we limit ourselves to +, - and () only. Then 0 will be unreacheable w/o using unary minus.
Also, for a value that you cannot produce, you need to be sure that you exhausted all possible approches, no?
I guess I just suck at number theory...