It took me a few looks over a few days to understand what this was about. Your comment above explains it just fine, but I didn't get it. You're basically discriminating between deployers and other subsystem services so the deployment batch building phase can start without waiting for those other services. The deployment batch building phase only depends on the deployers, so waiting for things like web connectors to start before beginning that phase is wasteful.
This makes sense to me as a general idea -- David, John, care to comment? Even if the entire server start gets redone and MSC-21 results in batching being eliminated, the idea of tracking when all deployers instead of when all subsystems are available, and then allowing deployments to proceed as soon as the deployers are done makes sense.
How much start time improvement did this allow? What was your config -- web container plus a ROOT.war?
I don't like the cast to ModelServiceActivatorContext. If we use this approach the param the subsystem activation API exposes should be ModelServiceActivatorContext (a better name would be good).
All this stuff (existing code included) needs a lot of javadoc; I had to think way too hard to understand what was going on!