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.
Once per encounter at the cost of a feat(with possible free skill) vs. every round at the cost of an at-will. Given non-human wizards only get one additional at-will, that's a high cost if there's no moment where Scorching Burst represents the best possible action.
This is also in the context that Wizards, if anything, should find it difficult to pick Encounter/Daily powers that don't target areas. Non-Wizards getting it once a day often have problems picking up area damage.