I think there is great value in having these calls. I personally join these calls to keep in touch with whatever is going in the developer community as I sometimes may be involved in projects that do not directly relate to editing code on OpenMRS core, modules or even JS client applications.
The reason for low attendance I think is the timing of the call. I am not sure how many developers are expected to join from South & North America where the time difference is not that much but I believe there are a lot of people involved in OpenMRS development from Europe, Africa & Asia. For most of them from the later regions the time difference is too big that the call is usually late in the evening, it gets worse for people in Asia. As a result most of people from this region end up choosing to join the calls when it is absolutely necessary, such as when a topic that directly affects what they are doing is being discussed (Myself being an example when I was working from Kenya).
One solution is to change the timing to accommodate most people, something like 0800 EST. The other is to have the call less frequently as suggested by @ssmusoke although I would suggest once every month instead of every two weeks because I feel like a month is an ideal period for the community to build the energy and enthusiasm required to be ready for the next call.