It all depends what type of foot pump Duo-Art you are talking about!

In the UK, foot pumped Duo-Arts (widely known here as 'half' Duo-Arts)
are actually remarkably common in the upright version, and even appear
in grands.  Some turn up simply labelled 'reproducing action' without
the Duo-Art name.  The works of most are 99% the same as a Themodist
Pianola, with just an additional accordion and Duo-Art tracker bar
added.  They are worth very little (if any) more than a Pianola.

I think these instruments, common in later days, were actually sold to
increase the sales of Duo-Art rolls to people who would otherwise just
buy 88-note rolls.  Or perhaps it could be considered as making a wider
repertoire available for very little extra cost.  Duo-Art rolls were
actually cheaper to make than Themodist ones because they didn't
require any marking after cutting (UK Duo-Art rolls didn't have words
or dynamic lines).

Pedal-Electric Duo-Arts are quite a different thing.  These are full
electric Duo-Arts with additional works for foot playing.  They have
a special Duo-Art box with extra accenting valves in them, plus all the
associated additional control equipment, and are significantly more
complex than the electric-only version.

These instruments cannot play Duo-Art rolls by foot -- the entire
'Theme' side of the expression system is cut out except when the
electric pump operates.  They were designed expressly to play Themodist
rolls by foot and Duo-Arts electrically.  A large segment of the
British market wanted to keep their options open!  These machines are
easier to sell than the electric-only version, so command slightly
better prices (both being much more expensive than the pedal-only
version).

You can imagine the confusion possible from having two instruments of
significantly different values which cannot normally be told apart in
casual descriptions!

When it comes to musical worth, of course that's another matter, and
one depending on personal taste.

Julian Dyer