Let me quote selectively.
Steve Dorner writes:
I just spoke to a fellow with a very interesting email proxy system
that unfortunately works by using such 'ed cruft in subject fields.
I suppressed my gag reflex, because I understood why he was doing it.
Most current MUA's hide all headers except a specific few, so if you
want to communicate anything to the end user, the only way you have
to do it is by scribbling on those few visible fields.
A further step that would probably be interesting would be to simply
remove such strings from subjects displayed in message lists.
Could this be generalized as a recommendation to MUA's? I dunno. It
seems like a feature that's useful a lot of the time, and harmful
occasionally (eg, when there is real information inside 's for some
Sounds like an arms race.
1. Proxy adds header fields.
2. MUA learns to show only a select few fields.
3. Proxy sticks information into subject using 
4. MUA learns to suppress .
In step 5, the proxy must stop using  and switch to something it's
more difficult to suppress.
The same situation applies to disclaimer/advert footers, I suppose.