[pclmulqdq]

Most professional violinists play in perfect equal temperament. I never got that deep into string playing, but I assume that a lot of study of "intonation" is actually about unlearning the natural frequency ratios (3/2 for 5ths, 5/4 for 3rds, etc) and learning to use the equal tempered counterparts (2^(7/12) and 2^(1/3) respectively).

However, there are a lot of times when you can make music more interesting and exciting by adding some pure thirds (equal temperament is off by the most on thirds, and thirds are very harmonically important) at strategic places. You just can't do this on a keyboard instrument.

[plaguuuuuu]

I think I find it a bit dull when violinists stick purely to ET. It sounds a bit less lyrical.

Otherwise the hard part must be choosing just the right pitch vs the other instruments.

For cadenzas or solo, do what you want I guess

[gnulinux]

I think this very much depends on the context, and being a good virtuoso violinist (or cellist, or singer etc) picking the right temperament for the right effect. If you're playing in an orchestra with many other instruments, you likely have to stick with 12TET. If you're playing a violin concerto cadenza, if you're playing a piece for solo violin, you likely want to play in just intonation as much as possible. If you're playing a piece for accompanied solo violin (violin + accompanying piano or orchestra i.e. sonata or concerto) then it very much depends on the moment and what sounds good for the music. Especially for an instrument like violin, which is extremely sensitive to every tiny expression performer can add, it's hard to make blanket generalizations. Ultimately, it's all about the artistic style of the performer, and composer's vision.

[musicale]

> You just can't do this on a keyboard instrument

Excepting split sharp/flat keys (as seen in some non-equal-tempered harpsichords and organs), or some electronic instruments/plugins which can dynamically vary the pitch of each note.