I can personally vouch for having a break section in your leader when fishing big game. Particularly important if you need a heavy tippet to protect against chafing from a gill plate or tail. I had a bft crash a fly that was dead-drifting close to the boat last year and as Murphy is always present my left foot was perfectly positioned inside one of the loose coils of flyline on the deck. Three seconds later it was tight around my thigh and if all I had was the 40# tippet I might have had some serious issues. Instead the 20# break section ahead of the tippet popped.
I was bummed but still standing on the foredeck.... OK by me!
And I belive the rule of thumb for flyline break strength is 35 - 38#