A level head is a far better attribute to have at any time in any sport.
Aggression leads to rash behaviour, and even worse a smack in the mouth if you get aggressive with the wrong person. there is always someone bigger and badder out there. I fought in martial arts tournaments around the country for many many years, and always found that the overly aggressive fighters (we call them players but whatever) were never the winners. ok they might take you by surprise, and score a few points off you in the beginning, but any level headed fighter will tell you that it is easy to pick points off people towards the end of a fight when their aggression has worn off, and they have knackered themselves.
What do you class as aggression though? someone who makes the snake off the break isn't being aggressive, just very fooking quick. if a team moves up the field very quickly it is not necessarily aggression, but good team work and a solid game plan.
a "hard" and "aggressive" team might have been more successful up to now where it was only a matter of one game being won or lost at a time. moving milleniums into the SPL/CPL style of play, it is all going to be about the teams with the better game plans overall. aggression will only get you so far, but the teams with their heads screwed on will always come out on top.