I'm unclear about the part about expensive queries to the data store. What cron/celery/quartz job would require a check to the data store for a check that wouldn't need to be done with posthook? It seems like if work depends on a decision made with data from my data store, that doesn't change based on what I use as a timer for the task. I'm not sure that I see a clear value-add here.
With a recurring job say every minute, the query triggered by the job to send out event reminders would be something like "get me all the events starting in one hour." With Posthook, you are able to start jobs only when needed and the query changes to something like "do I still need to send out a reminder for this event id."
