Sometimes playing it safe can be more dangerous than fighting on Instinct and hitting back. Like say you defend as if it was any other opponent and now your up against a world class guy, that will only put you in trouble. Better to punch back.
In my opinion you need to reach a high level in order to rely on defence. Footwork and circling will always work, But once he gets inside, it gets really tricky.
Reflexes, experience, fight IQ all play a huge part. Its not like you can just watch Floyd Mayweather and copy it. Thats harder than to just bang.