Many violin players sometimes play viola. To do this we have them learn to read alto clef, the clef in which viola music is traditionally written. Why make them learn? Instead we should start printing viola music in mezzo-soprano clef:

Playing a mezzo-soprano clef part on a viola would require the same fingering as playing treble clef part on a violin. Wouldn't that be nice?

(Now that music printing is computerized, it shouldn't be that hard to make available viola parts in both alto and mezzo-soprano clefs.)