Technically,  when he was in the band there were five members so that would give a semantic claim to the title.  But the term is obviously intended to reflect the person's contribution to their music.  From most of what is written about Stu, he wasn't much of a musician and his departure was inevitable.  He arguably did contribute to their sense of style and their look, especially the haircut.

Not a musical talent- as stated below, Stu was more of a friend than anything else. 

Poor Stu and a very short life and could not play an instrument. His death was very sad but he was a friend more than a contributor to the group.