What do famous violin players have to do with the viola? Well, many of them played it and play it. Until the 20th century it wasn’t common at all for a performer to play viola as his first and only instruments.
With the important exception of Carl Stamitz and Alessandro Rolla who, in their time, were famous more as virtuoso viola players than as violin players, the viola was not considered a suitable instrument for a virtuoso, yet apparently a number of famous violinists liked it enough to play it occasionally and still do it.
It’s funny, it seems to me a bit like having a wife (or husband) and a lover. Famous violin players used to “marry” the violin but every now and then liked to have a go with the viola. It just wasn't fashionable to play viola. This situation has gone on until someone eventually decided to “marry” the viola and play it as their first choice instrument. These were Lionel Tertis first, followed by William Primrose. Nowadays it’s normal for a soloist to choose the viola as his/her instrument.
William Primrose didn't approve of violin players playing viola, he thought it was a sort of 'unfair competition', that they generally didn't have the sound depth and quality of a viola player who had spent most of his time improving his viola playing, and therefore didn't do a good service to the viola cause. He used to call them 'moonlighting viola players'.
It is true, yet to me it also seems like a violin player who plays viola is in a way paying homage to it, acknowledging its qualities.
In Primrose's time there weren't many good viola players to make a decent comparison with good violin players. Now, with many more excellent viola performers, this comparison may result more favourable to full time violists.
Anyway, here are only some ancient and modern famous violin performers who played and play viola and wrote for it. I'm sure there are more, but I have to dig a lot to find them, I'll add them whenever I discover them.
The most famous violin player played the viola too!
You'll find some information about each of them in their pages, just click on their names.