It isn't subpar as something you'd use once per combat. It is subpar when it is either you use Scorching Burst or X and your opponents are spread out.
I don't see how having the ability to toss a burst 1 once/encounter can be /better/ than being able to toss a burst 1 every round. That's just absurd on the face of it. Are you trying to say that there just isn't much call for area spells? Area spells don't just do damage (or put effects) on multiple enemies, they also bypass concealment and cover, and they act as area interdiction - that is, they keep enemies from staying in formation or bunching up to concetrate melee power on your front line. If you assume you always succeed at that, whether you actually have an AE or not, I suppose the value of area attacks isn't that great, anymore.
I don't think that's a safe assumption, though. A lot of monsters benefit from being adjacent to thier allies or ganging up on the same PC (over and above the usual advantage of concentrating damage on one victim). If monsters stay spread out, they're giving up those advantages - so well-played ones won't disperse unless forced to by the controller. After dispersing the enemy, the controller no longer dishes striker+ damage, but he's succeeded at his role.
What if one were to alter Scorching Burst as follows:
Damage: 1d6 fire damage (plus see effect)
Effect: Target takes additional fire damage equal to the caster's Intelligence modifier at the end of its next turn, unless it spends a move action to extinguish the flames, which is automatically successful with no saving throw needed. This damage persists indefinitely until the target spends the required move action to end it. Additional applications of this effect from multiple castings of Scorching Burst do not stack.
Interesting idea.
The more conventional way to write it up would be:
Hit: 1d6 fire damage, plus continuing fire damage equal to your INT modifier (target my use a move action to end).
The problem with the conventional mode is that continuing damage happens at the start of the victim's turn, so they'd almost always take the damage once. Actually, that's not so much a 'problem' if you want to upgrade Scorching Burst. Re-writing continuing damage rules for a specific power is always an option in an exception-based system, but IMHO, the game runs smoother if you try to avoid such things.
Or the secondary fire damage could be based on Wisdom, or Dexterity, or a flat amount, or another 1D6, or whatever. I'm not sure what would be best.
Basing it on DEX would make it a 'wand wizard power' - basing it on WIS would make it an 'orb wizard power' (and they already have /two/). A flat amount - 5 is most common, 2 is apropriate for a 'weaker' power - wouldn't be out of line. You might want to let INT add to the base damage, though, if you go for a flat amount, like 1d4+INT +2/continuing, OR:
Or maybe return the hit damage to 1D6+Int and just make the secondary damage 1 point per round. I dunno.
And should the effect happen only to those hit, or to everyone in the area? I'd think the latter would make it very potent, but maybe too powerful.
Only those hit, so it doesn't step on Cloud of Daggers.