I always bring up the question: "How many of you go home, open up your physical mail and then stick it right back in the mailbox?" Imagine the US Postal service running around replacing shoebox size mailboxes with 50, 100 Gallon or swimming pool size drums to accomodate the laziness of recipients that don't want to bother sorting out what's legit and what's junk and filing away the really important stuff. You wouldn't do it at home so why at work is it OK?
E-mail is no different. If a user is not sure what they are supposed to keep for 4 years or what other compliance issues need to be met, then they need to learn and be held accountable for how they handle their correspondence. The mailroom guy would never be held accountable for an executive throwing out something he should have kept yet an email admin could easily be fired for the same.
I've found that there is never a solution that everyone likes but explaining the pro's and cons of .pst files, limits, and filing of messages - all user/management training goes a long way. Ultimnately THEY have to sign off on a policy before it's implemented and if they go with a solution that's very liberal, they are aware of the caveats and risks.