FF, I understand what you are saying, but I have never had to use your language when booking multiple cruises onboard. We rarely take just one cruise, typically 3 consecutive. In either case, why we do it this way is personal. Regarding nonrefundable OBC, it is always rolled over until the end of our last "segment" "cruise" -- we don't have to try , it is automatic.
Re: booking SM excursions -- we are able to do this each "segment" whenever the date opens for booking (365 days before each cruise) , just as we do with specialty restaurant reservations. We are restricted to those opening dates, while if we booking as one cruise -- we could do it all at once. We don't mind the restrictions of having to do this 3 times.