ljcAs someone who's played mandolin for some years and recently decided to try and add the fiddle to my repertoire, I'd query your comments re practising fiddle left hand positions on a mandolin.
Firstly the scale lengths of most mandos and fiddles are different and so the fingering positions are not exactly the same (though the theory is).
Secondly the angles at which the necks are held are different and this will affect significantly the ways in which notes are "accessed".
Thirdly, when was the last time you compared relative prices of entry-level fiddles and mandolins. I got my hands on a very playable fiddle (complete with case, bow and resin) for £60 at Hobgoblin the other day. A mandolin of comparable quality would have set me back at least £150-£200.
And then, of course, the whole thing about the fiddle is that the left-hand note positions are actually pretty easy to learn. Much more difficult are the other left hand techniques (slides, rolls, vibrato etc.) for which practising on a mandolin will not help.
Not to mention the right hand!