In my opinion it's the artists own instinct and feeling for the work that is lost for drawing by numbers; a mechanical and rigid way of replicating reality that becomes kind of dead in my view. I'm not saying that all work rendered using the grid method is like this but artists who use it can lose what is special in the work that only a freehand drawing can achieve.
I believe in the long term you can achieve greater drawing without using the grid method by only using your own observational skills. It might be harder but it's a lot more satisfying and you learn more from it in the end.
This is only my opinion so please feel free to disagree.