Have you seen Isner lost against Kohlschreiber yesterday at the US Open? Isner lost because he was too defensive and let Kohlschreiber make the points and he did. It's classical: there are more examples on topranked players who got afraid/or didn't play aggressive. Actually looking at those matches you can learn a lot.
So an aggressive way of playing, knowing your strengths, weakness opponents (Brad Gilbert: every player has a combination to crack) and a mental toughness are more important things to work on than being offensive or defensive.
And as said: during rally you need the skills to switch between defensive-neutral-offensive...
quest to the perfect superduper racket...