According to the Parse Requests and Dispatchers help topic:
Upon receipt of a request, it will generally queue the request, possibly disposing of similar previous requests that are still on the queue.
It doesn't appear that the default implementation, ThreadedParseRequestDispatcher, disposes of previous requests when a new one is queued. That is the behavior I want, because it doesn't seem useful to execute any but the most recent parse request.
Do I just need to implement my own IParseRequestDispatcher?