The only reason I can see for hesitation is needing to be airlifted for medical reasons- but that's a pretty big 'if'!
Or missing the ship and needing to fly somewhere to catch up with it. You can't fly to a foreign country without a passport, and you can't fly to another US port to board.
... the Passport cards seem like a gimick that often is useless. Just get a passport.
I wonder how long it takes to get a temporary passport from the embassy when you are under a medical emergency? I don't know if I could handle that kind of stress especially if you have kids involved. But that is me... so, I'd definitely get the passport but understand the cost is an issue especially if you will never use them again. So, I'd still choose either between the passport or crusing with birth certificates.Now if there is some sort of emergency, you need to contact the US embassy immediately. And be prepared to pay for a passport.