Starting 12/12/2013
DCL has issued the rule that your On Board Booked cruise must be completed within 18 months of the day you book your (dummy) OBB. At the same moment the reduced 10% deposit was limited to cruises of 7 nights or longer. Within that timeframe of 18 monts you can change the cruise you want as many times as you wish. Keep in mind that for any of the reservation you must PIF before the PIF date even if you wish to move the date again or the booking will be canceled an the OBB discount lost.
This means that if you are definitely not planning to book a cruise within 18 months there is no point in booking while on board and the DCL person you spoke to was right. However in older threads on the boards than December 2013 you will see the old rules discussed.
The DisneycruiselineBlog wrote this article on the subject:
http://disneycruiselineblog.com/2013/11/onboard-booking-offer-policy-revised-use-lose-18-months/
For info purposes the OBB request form can be found here:
http://disneycruiselineblog.com/wp-content/uploads/2014/07/DCL-Onboard-Booking-Offer-June-2014.pdf