This may or may not be true.  Certainly, a great script will never save a good story.  And an average script won't do anything to hurt a brilliant story.  Personally, I would say you should be great at both.  Think about it this way.  If you or your agent submits this script to a studio, what is the reader going to think?  The script can be good, though the story is bad.  They won't get past the first ten pages.  However, a story is absolutely fantastic, new, uniques, exciting, clear.  But the script is average, so they still only read the first ten pages before passing.  Remember, the correct term is a "screenwriter."  We write for the screen, not the script.  Which means that we have to be doing it better and cleaner than the other guy, 100% of the time.  Come up with great stories, but learn HOW to write them, not just WHAT to write.