Yeah, I'm a little concerned about that last line too.
For 1 Refresh, your Stoicism covers both physical and mental attacks. That's much cheaper than buying Toughness and Stoicism.
Plus it might be possible to game the Catch. Take a Stoicism Catch that's completely unobjectionable normally, and make sure that it can't be satisfied by any physical attack. Then take this, and have Catchless Toughness at a discount price.
And now that I think about it, the fact that this is optional is a pretty big advantage already. If my entire mental stress track is full, I can start taking hits on my physical stress track.
So yeah, this is worryingly powerful.
On the other hand, you have to invest like 5 Refresh to get it. So it's not totally nuts. But I'd still hesitate to allow this.