One comment to add to all this--a friend of mine who is a professional
orchestral clarinettist visited me recently after shopping for some new
instruments in Europe.  He owns several clarinets in various keys, and
told me that (as Deryk Barker mentions in his posting on this subject)
that tonal qualities can be different enough between differently pitched
instruments to warrant having them around.

For example, in Beethoven's time, clarinets in C were quite common, but
by this century they weren't seen much at all.  The instruments in A and
B flat were considered to have superior tonal qualities and could cover
the range that the C instruments could.

However, my friend told me that he had recently bought a new modern
clarinet in C, and that these instruments were "coming back".  Part of
the reason was HIP-influenced, in that the tonal color of the C instrument
was sufficiently different to warrant its use nowadays, especially for
music of the earlier part of the 19th century.  And as we have seen, what
was considered musical "truth" for the earlier 20th century has taken its
share of bumps over the past few decades.

I don't know if this is a general trend, or if all other clarinetists in
a similar position buy the idea.  But based on my friend's word, I'd be
a little slower to jump to the thought that it would be "progress" to have
all instruments become non-transposing.

Bill H.