Some thoughts:
We can scarcely blame them for not being fully MIME compliant at this
stage, when the spec is still at the "draft" stage, and so soon after
the introduction of MIME-capable software.
Using uuencode rather than base64 seems like a reasonable strategy
for the interim while MIME-compatible mail readers are not widely
available. This actually provides an incentive for the, ah, "readers" of
alt.*.pictures to get MIME-compatible news reading software (so they
won't have to manually re-assemble and display pictures), without
leaving those who don't yet have such software out in the cold.
(Of course, such MIME news-reading software has to support x-uue
encoding in addition to the "official" MIME encodings.)
So I think those who are using MIME in this way are actually doing
the net community a service.
My suggestion would be to try and promote some sort of "flag day"
(say N months after RFC-MIME becomes a proposed standard) after which
people are strongly encouraged to use base64 instead of uuencode.
Keith