The downside is that you have to fly into one city and out from another, which can be more expensive than roundtrip air tickets. Or if you live at the point of origin, you need to buy a one-way ticket back home, which is usually quite a bit more than half of the roundtrip fare. Another downside would be the length of the cruise - if you are enjoying it, it will end WAY too soon!
My husband and I are doing a similar one (L.A. to Vancouver) this May, our first time on this type of cruise itinerary. It's an experiment for us, but one we're looking forward to!