JeffJetton
Prolific poster
Many (perhaps even most?) 60-bass accordions voice their 7th chords without the root, so that they can do double-duty as dim chords. For an Edim triad, for example, you could, as you suggested, play the C7 chord button with the E bass button (counterbass of C), and it works perfectly. Or play the regular E bass with A7 to get an Edim7 (without the flat 5, which is the same as what you'd get on a 72-bass).Dims are a "nice to have". There are some folk tunes I play that use them, but they can be worked around - e.g. one tune I play is notated with E˚ but a C7 works almost as well. I was looking at 12x5 and 16x5 bass layouts and just thinking I wouldn't want to be six months down the line and wishing I'd got something with the dim chords.
That said, the weight/size/cost difference between 60-bass and 72-bass isn't usually that much. And there are a few small advantages to having separate, dedicated 7th and dim7 buttons.