I completed a 6 week beginners course 3 weeks ago.
I agree that many people in the course do not become members but I don't see this as a negative thing. Since the course runs for a defined period of time it gives people an opportunity to sample Aikido even if they know that they do not have the opportunity to commit to it long term. This helps to "get the word out there" about your dojo and brings in other beginners who WILL become long term members. Our beginner's course also did not require investing in a keikogi. We worked out in sweats. This helps get people in the door so they can decide if Aikido is right for them since there is little initial investment to lose.
In our beginners course we focused on basics and got an opportunity to learn the customs of the dojo in a non-threatening environment. Since we were all beginners there was no sense that you were the only one in the room who did not know the customs and exercises. Everything was carefully explained in the beginning and the pace picked up over the term of the course.
When it came to techniques, we stuck to the basics. Each technique was demonstrated for us 5-6 times with important points emphasized so we would have the opportunity to take in as much as possible before pairing off with a partner. We would train for a short time before stopping so that the instructor could point out further details about the technique. This was immensely welcome. During these pauses the instructor would point out what the class was doing well and then point out elements of the technique to focus on that were being overlooked or gummed up. Then we would grab new partners. Each technique started at the first step and then more elements were added throughout the evening until we were performing the full technique. Most of all, the instructors were charismatic, patient, humorous and conveyed the spirit of the dojo well.
Higher ranked members were encouraged to attend the beginner's course. These members were excellent Uke and had a lot of good advice during practice that was offered without ego. I believe that they also benefitted from this exchange. Additionally, I managed to form a bond with some of these senior members and they have continued to show a personal interest in my progress. Their attendance in the beginners course was indispensable.
After the 6 week course concluded we had 2 weeks of "tweeners" courses. The beginners course taught us Ukemi and the basic techniques while the tweener courses further prepared us to join the general classes by empasizing us insight into what to expect. For example: one of the most important things I learned was how to "beat feet" across the mat to get to a good partner so I wouldn't end up running in circles like an idiot. The tweeners course also picked up the pace and intensity to better match the general class.
Personally, I believe that a beginner's course is the best thing that a dojo can do to encourage a larger membership. It provides an environment where people can grow into your Aikido community and creates a welcoming first step for people to "try on" Aikido to see if it's right for them. It also seems to help shape the community within your dojo and teaches Sempai to take an active role in their Kohai's progress.