In my opinion, I think everyone should attend orientation if it's possible. I know it helped us a lot to get a perspective of what the process is, and getting to meet a couple birthparents and a family that have adopted children was priceless for us. Their stories really helped us understand what people go through, more than reading it in an article or book (although the books I've read have been very helpful too but there is nothing like hearing a story firsthand).
That said, if adopted families are needed and there are just a few couples ready for orientation then it seems that letting them be tweeners would be the best option. Skype would be a good option to go through the same orientation process for these couples. Then they get to meet you and you can start to know them more than just what was sent in on paper and in photos. Or maybe they can start the process and should attend the next orientation...even if they are already matched or have taken placement then this is a story that they can share with the group. If it means you have one or two more couples in orientation than normal, I still think that's ok.
I do think orientation should continue if its possible and look forward to coming back to an orientation weekend as againers in a couple years!