What sort of legitimate communications would amateurs be doing "on a
regular basis"?

Really, any radio communication an amateur radio operator does can be
done just as well through other communication services. The one main
problem with amateur's is that they would rather debate the code/nocode
issue instead of making advancements the art of radio. In the 80's if
you wanted to talk over the world there was three way of doing it,
regular mail, the telephone, and ham radio (the first two you pretty
well had the know the person or a member of a pen-pal club). ham radio
was the main means to communicating all over the world. Then suddenly,
out of the blue technology oversteps amateur radio and now all a person
has to do is get on the internet and chat. the only true thing amateur
radio is good for now is emergency communications and that's about it.
Amateur's have not done anything to advance the radio art.
97.113(a)(5) was writing to protect the other radio service who
apparently felt threaten when that rule was written (which was a long
time ago) by amateur radio operators. (Amateur's at one time competed
with the commercial radio service and military) what I believe the FCC
should do is ever two to three years go over the amateur radio rules
and ask themselves do this rules need to be updated to keep up with
technology? Some of these rules have been in place since the 1930's and
haven't been changed one bit.

Encryption doesn't by it's nature enable frequency multiplexing;
that's something else entirely.

Somewhat true

Spread spectrum allows multiple people on the same frequency at the
same (by human senses) time, and is legal

The problem with Spread Spectrum is that although it is legal in the
amateur radio bands it is still tightly regulated, for example Spread
Spectrum is only allowed on 70 cm or higher, and is limited by power
and limited on system (ie there is I believe three forms of spread
spectrum and amateurs many only operate two of them because third one
violates the FCC rules as written) and the FCC is under the belief that
ALL RADIO SIGNAL cross the state line and travel into foreign
countries. As I stated although it is Legal, it very limited due to
rules and regulations as written.

Todd N9OGL