I'm surprised that's the response you got given the text here.

http://travel.state.gov/content/passports/english/passports/information/card.html

Get the book. Even if it was usable (which I'm pretty sure its not) why limit yourself?

EDIT - If you are taking a ferry from St. Thomas to BVI, then I can see why they responded the way they did and it may be accepted. If you were to fly into EIS, then its no good.

Here's something to consider - what if there was an issue that required you get home quickly? And flying out of EIS through San Juan or even to St. Thomas was the fastest method? I think you'd be SOL with the card as it is not good for air travel. While the book is more expensive, its a minimal cost over the 10 years its good for and covers you regardless of where you are.