Last week I spent half a day converting myself from a decades worth of old procmail and thunderbird rules to dovecot 1.2's implementation of sieve.
While I liked the clarity and simplicity of the language... and the power of dovecot's imap server... and having stuff in Maildir format...
The idea of piping your now-filtered email through another program is missing from that version of sieve! It's a one stop only filter.
So I lost the ability to do additional processing on the email itself as it came in, for example, when I get email from certain people I used to feed it into a speech synth (instead of "You've GOT Mail", I get "You've got mail from Joe")
If sieve had a "pipe" plugin so I wouldn't have lost this crucial facility of procmail I'd be a much happier camper.