Of all the instruments of the orchestra, the viola is the one whose excellent qualities have been ignored for the longest time.
It is just as nimble as the violin, the tone of its low strings possesses an attraction all its own, its high notes dazzle through their sad = passionate expression, and its sonic character in general, of profound melancholy, differentiates it clearly from the other string instruments.
Nevertheless, it has been left out of consideration for a long time, or mostly been consigned to useless activity without content, doubling the bass voice in the higher octave. A number of reasons have come together to bring about this unjust use of this noble instrument.
First of all, most of the masters of the past century […] did not really know what they should do with the viola. […] Thereupon it was unfortunately impossible to write down any more significant spot for the viola that would have required even an ordinary talent to execute. The viola players were always selected from the ranks of rejected violinists. If a musician was incapable of properly performing as a violinist, then he took up the viola. This is the reason why violists could neither play the violin nor the viola.
I must even confess that this prejudice against the viola part has not been completely eradicated even in our time, for there are still viola players in the best orchestras who cannot cope with either the violin or the viola. However, one has more understanding, from day to day, for the awkwardness that results from the toleration of such people, and thus the viola is being gradually entrusted only to skilled hands, like the other instruments.
(Hector Berlioz)