I see two applications for a filtering language:
(1) final-delivery MTA filtering
(2) client filtering
With (1), the filtering happens automatically, which is great if you want
to run a shared folder which doesn't need babysitting. With (2) you're
likely to get a lot more power and flexability (and have no problems
discovering supported extensions).
Having the IMAP server do filtering is the worst of both worlds (limited
features, and requires babysitting).
There is no reason that (1) and (2) couldn't use the same core language --
and I think that would be a good thing. However, (1) and (2) do need to
be stored in different places (at least in different ACAP datasets or
options) as they provide different levels of service.