I think we all say stupid things at times that we'd rather not have said. Unlike us however, Justin Gimelstob gets the privilege of saying them on national TV.
What he probably meant was that for Murray to get to the next level of greatness that the other slam winners of this era have attained, he has to be able to have the option of going up the line more often with his forehand. The danger with not having nearly as good a pass up the line as crosscourt is that people can lean on the crosscourt pass.
Now the rest of his game is just as strong if not stronger than the current world no.1, so developing that part of his game seems to be prudent.