The Coleridge-Taylor Violin Sonata that John David and I are playing tomorrow night is loaded with sixths. Violinist-composers tend to load up their music with sixths because the sixth is such a harmonically rich interval. It is simply loaded with overtones, some that can be heard, and some that can't really be heard distinctly. They can be felt though, by the person playing and the people who are listening. It is rare that a microphone can pick up the full array of overtones and difference tones. These are the things that give texture to the music and contribute to the personal quality of an individual player's sound.
I like to think of those overtones as sheep that I am herding. Getting them to line up and come with me is a task that is gentle, yet firm.
The overtones in the interval of the third are a bit more rambunctious, and sometimes playing successive thirds is more like herding cattle than herding sheep.
Sometimes, in some keys and in some registers playing thirds is like trying to herd wild horses.
Practicing scales in double stop sixths and double stop thirds has its benefits beyond simply knowing where the notes are and being able to sustain pitches on two strings at the same time. I believe that it enriches the sound. Every key is also a different "herd." The traditional violin keys (G major, D major, A major, and E major) are actually far less stable (or with more wild horses) than keys that have more sharps and keys that have multiple flats because the sympathetic resonance of the open string and its interaction with the other pitches is an added factor. Throw in an open string, and you need a lasso!
The surprise treat for me in this adventure in sixths is the key of G-flat major. It is unusually rich and warm. Your fiddle will thank you for being brave enough to enter into that forest of flats. Actually, I think that "forest" is more a term I would associate with sharps. Perhaps it's a forest when you think of the key as F-sharp major, and it's something else when you think of it as G-flat major. A swamp perhaps?