I realised today that many of my favourite artists are both strong single and album bands. And it was very remarkable that in mid-sixties many of my favourite "album"-bands released great singles where both sides were not in albums (for example Beatles, Rolling Stones, the Pretty Things, Traffic, Jimi Hendrix, Cream, Pink Floyd, The Who and in Finland Blues Section, Wigwam and Tasavallan Presidentti). It was in the seventies when albums came more important than singles, but there were also exceptions. When Jethro Tull released their great entity "Thick as a Brick" they also released same time an ep "Lifes a long song" where were 5 average songs. And for example Hawkwind and Amon Duul 2 released singles whose songs were not in albums. In new wawe it was also very typical that singles songs were not in album (for example Joy Division, Bauhaus, the Cure). Today singles seem to be only promotion for the albums. And yeah, SY are not very strong single band (many of their singles have a-side from album and not very good b-side, Genetic is almost only exception), but they have one great single that are not in album: Flower/Halloween.
By the way I really love many prog entities like Genesis Lamb Lies Down on Broadway and Yes Tales From the topographic Oceans.