Yes, we did just this.
Why do it--we couldn't get to port until Saturday night and we wanted a 7 day cruise. In addition, it was cheaper for us than a 7 day cruise (this was only by about $150, and would not have made the decision for us.)
If you have a selected cabin booked, you can get the same cabin for both legs, which makes your life much nicer. We had a cat 12 booked, did not get upgraded, but were required to change rooms. I asked the TA to call
DCL about this and she supposedly did--response was that both sailings were full, etc., couldn't be done. We bought an upgrade at the port for the first leg, so I didn't really care about needing to move.
We were treated like royalty. Before the second leg, we got a call from Dining Services asking if we wanted Palo or had any seating preferences for our second cruise.
That said, I wouldn't choose to do it again. It is the same shows, same menus, same ports. Everything on the short cruises, especially the 3 day, is very crammed together. You miss the Art of Entertaining series, the character breakfast, etc. Unless there is a good reason (like we had) for doing a 3+4 or 4+3 I'd choose to do the 7 day.
We actually had a great time on the second embarkation day. We met another guest in the terminal who was doing the same thing and spent the 4+ hours together. It was a totally good mix, so we arranged to be seated together for our dinners for the remainder of the cruise.
Under normal circumstances, the services on the change over day will be very minimal for about 3 hours. You can hand in the Cove Cafe (open to all ages for those 3 hours) with minimal snacks. You can use the pools when they have finished cleaning them, but there is no lifeguard and the slide will be closed.
As to B2B 7 day being "very expensive," it again depends on the cruise dates. You can do October 3 and 10 or December 5 and 12 relatively inexpensively. If your selected dates include a high season date, it will be a "very expensive" trip. If they are both low season, you can get great rates.