The defacto standard for sending anything but plain-text-English
emails is MIME. Ascii Armour is from the time that UUEncoding was the
hip thing. The mailreaders of 99% of the internet users today already
understand MIME, they don't understand AA, so adding support for
PGP/MIME to them should be trivial.
I tend to agree at least for corporate use since Eudora, Exchange,
and ccMail all understand MIME which would seem to simplify plug-ins.
AsciiArmouring is not necessary for PGP to operate properly, rather it
is a "nice" but is ready to be considered a hysterical element along with
TekHex and UUencode (which could also be easily supported).
However I must agree that the prevalent mechanism today is Base64 encoding.