But really, a raid by definition requires a large group of people. Thats the difference. 5 man is small and hardly considered a raid.
I guess the real point though is the request for longer dungeons which is fine except I would keep them out of LFD randoms. Maybe have them drop half a tier above heroic dungeons or something to compensate for the length and commitment.
I think many would probably think BRD, UBRS, Kara type length for 5 people which would be great but the incentive to do them would be minimal and anything more than minimal would be problematic.