Externalizing the internal logic of the app's message processing pipeline just does not seem like a good idea.
( Do I sound like a speaker at an Enterprise Cloud Storage Scalability Clustering conference? )
The Tasker Event API is amazing (I was able to get the plugin to work in two days, while managing the rollout at the same time) -- but it does have some awkward parts, and they do come up in some edge cases (I hope they really are that, edge cases). For message rules though, those could be a real problem.
And one more thing:
People ask about message rules *in the app*, not realizing that it's just so much better in many ways to configure them up on the server, *in the mail service*. They just may not be aware of this option.
So not having message filtering in Aqua might prompt some to discover the better way...
...and might prompt others to post angry 1-star comments in Play