- You don't have to write logic to figure out which clients need to be updated with some data. You just run the queries that you need to to generate the data for the client, and then RethinkDB sends you an update only if the result of that specific query changes.
- Having the thing that modifies the data send the updates to the connected clients becomes increasingly difficult if you have multiple application servers. Then you would need to set up some separate message passing / broadcasting infrastructure if you also want to update clients connected to other servers. RethinkDB takes care of "passing the news around", even in a distributed environment.
- RethinkDB supports changefeeds not just on the raw data, but also on transformations of the data. Not all transformations are supported yet (for example map reduce queries are not), but our goal is definitely to support changefeeds on pretty much any query. Just knowing that the underlying data has changed isn't enough. In a traditional setting, you would still need to either recompute the whole query, or implement your own logic for incrementally updating their results for every type of query you want to run. RethinkDB updates query results incrementally and efficiently.
(I work for RethinkDB)