I think that's the point. It deals a minimum of 3d6+2d6/level above 5th, if you leave immediately; not bad, but not on par with fireball, either. But a canny wizard/druid/psychokineticist will use it in circumstances where leaving is more difficult. Say by summoning a couple of fire elementals to sit in the sphere with 'em.
I don't know, do fire elementals need to breathe?
Everything within this sphere automatically takes 1d6 points of fire damage per round plus 1d6 extra per caster level after 5th (maximum 20d6)
Should that have a maximum of 15d6? Or does this scale up to epic levels?