Purchasing Future Cruises on A Cruise

[gerryuk]

Just off a transatlantic from Barcelona to Rio with Costa. I was surprised to see the department that you can book future cruises with on the ship, was very busy daily, in fact on some days they were queuing to get in the door. So, what is the deal with buying future cruises on the ship as opposed to buying online or with TA? Are there good financial deals to be had buying your future cruise this way or are you better off to go online?

What is the situation with solo travellers purchasing future cruises this way, Costa and some other lines will tell you that the rooms are no longer available when trying to buy solo tickets in their cabins via the internet, but the rooms are available if booking for 2 people, will you get the same response if buying on the ship?

Has anyone purchased future solo cruises directly on the ship, what was your experience?