fretted instruments are generally tuned in equal temperament.
the fiddle is unusual in so far as it requires the player to adjust the tuning very quickly as the player plays.
if a fiddle player wishes to play with [for example]with a harpsichord in mean temperament]he/she can,alternatively a fiddle player can also play with a concertinist in equal temperament.once you start fretting the instrument,you are limiting its capabilities.
[IMO]anyone who frets a fiddle does not understand the instrument. if they cant play in tune without frets,they just need to work on their intonation,[imo]frets are not going to solve that problem.DickMiles