Erik_Squires gave you a good summary of the advantages of multiple transistors. Another advantage is that, for any given output, each transistor can be run more gently which will improve longevity.
There is a possible downside to parallel, multiple transistors vs. just a pair per channel. Some designers believe that multipole transistors will muddle the sound and reduce transparency. Dartzeel once sold a $20,000 amp based on this idea that had only two transistors per channel. They have since upped the count to four per channel in their $50,000+ updated version of this amp. Nelson Pass' First Watt amplifiers also use a minimum number of transistors in both input and output stages. I've heard several First Watt amps, and I had their J2 in my system for a couple of weeks (borrowed from a friend); it was one of the better sounding transistor amps I've heard (I am mostly a low-powered tube fan).