You can't be serious. OBC can be used for ship's excursions. If you can book these online before your cruise, you should be able to use the OBC you're being given rather than having to shell out $1000+ upfront, and then requiring the excursion staff to go through the trouble to cancel, rebook and process transactions to your credit card...for everyone who booked excursions online.
Azamara allows you to book excursions online using your OBC. Windstar should, too.