From what I understand, you can't punish someone after blocking (unless there are some cases that you could, but I don't know that much) and you can punish, at least in most cases, after a parry, be it a light attack or a guard break (I am aware that there are some cases that you don't get a punish after a parry).
So, if I am correct (at least partially) you would be missing opportunities for punishing your opponents if you do not attempt parries.
Being defense the strongest aspect in the game, not getting a punish after blocking the opponents attack it's preventing defense to be stronger and having to risk parrying (assuming there is a risk, if there is not it must be fixed, like not being able to cancel the heavy attack that initiated because the opponent feinted) to get a punish is what is letting the offensive player to open up the defensive one (among other things)
Then again, there is a lot I still don't know about the game, so I'm probably failing to see somethings