That means with 1 ship he behaves like a 2-card, with 2-ships a 3, etc.
You need more than 4 ships in hand to make him a super stong card. On the other hand, with a Def+1 order + him, you have a def of 3 against a Martell player and can still keep your Mace. Tyrell to get his fleet wiped away needs to either face the Red Vyper (that may happen but it can only be used once so it is risky) or to be beaten twice at least (in WSS and Redwyne Straights).
I therefore do believe it can be a valid card to help defending, even if with no mustering resisting will be hard, with one mustering in the first four turns you should at least survive for some time.
Anyway, you are right, having this option is probably a long shot. Admins, should you see this, please keep that somewhere in mind. ^^
Would be happy to know results if you use that at home, Franc (and Frode too).