Sling OptingServlets allow a servlet to be registered to a resource type and decide if its going to handle the resource. This sounds good, except that there is only one slot available per registration. If you have a resource type X, method GET and you want to have 5 Servlets registered to optionaly process requests against resource type X with no selectors and no extensions, then you cant do it.
The solution is to have a single Servlet registered as an Opting servlet per resource type, and have it manage a set of Servlet implementations that opt on that single registration. The only thing that you need to be careful about is to create a service interface that Sling wont pickup and register as a servlet, otherwise the likelihood is it will overwrite your Servlet multiplexer, and with 2 identical registrations of the same request/resource patter, its random which one will be active.