@Sol and Ebor: Triangles and drums: most of the orchestral percussionists I have met have done a long hard musical training (and probably happen to play a melody instrument as well, but have just just chosen to play percussion). Your triangle player will probably flit around a vast array of different percussion instruments in the course of an evening's concert. Occasionally, if they are really short they might draft in one of the 2nd violins to play a single (well-placed?) ting on a triangle!
As for singers: yes, the human voice is a fine instrument too. I can think of several singers I know who do not read music, yet are capable of producing fine harmony and polyphonic arrangements without ever being able to write it all down in standard notation. So, yes, singers, are musicians too in my book.