If our eyes only saw frames (IE: 30 images a second), like a single lens reflex camera, we'd see images pop in and out of existance and that would really be annoying and not as advantagous to us in our three dimensional space and bodies.
In the Vienna of the second half of the 18th century, in addition to Haydn and Mozart, three symphonic composers, namely Leopold Hoffmann, Carl Ditters von Dittersdorf and J.B. Vanhal helped further the development of the symphony.
Well, it depends on the symphony itself. Here's a symphony by W.A. Mozart which calls for relatively small forces. Symphonies never have singers or a choir, except of course, when they do. Beethoven started it by putting a choir and four soloists in the last movement of his Ninth Symphony in 1824. Since then, a few composers have called for choirs in the symphony to communicate text, though it is not common. Over time, the symphony grew in size to become an epic dramatic expression.