"The old general tune books that I have (and I exclude the ones aimed at a particular instrument like the small pipes) do seem to contain tunes with a wider span."

Like you, Will, I've spent time with old MS collections and been strcuk by the frequent use of flat keys. But I think you're talking about more recent collections here. Those little landscape format books that EFDSS published in the 1950s have tunes in A, C, F and Bb (I have '100 English Folk Dance Airs' in front of me). However, back in those days a dance band would more than likely have been led by a paino accordion. The whole aim of the English Country Music Revival of the 1970s was to get away from the piano accordion and non-English repertoire, and to paly tunes like Oscar Woods did.

Harmonium Hero is right in that some melodeonists at least eventually find the limited flexibility of their instrument frustrating, and go either for a diatonic/chromatic hybrid, a three-row, or a machine with extra buttons and chords (I've a few of those myself). There's always the feeling, though, that you're losing the essential simplistic beauty of the instrument - which is why other players go the other way and switch to one-row. Mind you' I've heard the great cajun player Steve Riley execute key changes in mid-song that really ought to be technically impossible on a ten-button one row!