@SageofMusic
A single attack is not so much filling squares with smoldering flame you have to move out of, it's a single scorching blast of fire erupting from the origin square. "Zones" were created to model what I think may be what you are envisioning where the magic fills the area for a certain amount of time and must be moved out of to avoid damage, or whatever the mechanic on the power is.
If you think a spell like fireball is very short-lived like I do or if you're just with me so far, then it's just a matter of putting something in between you and the origin square. A shield, a large pack, a blanket, bedroll, something of the sort. That's assuming fighting in a clear plain. If there is cover sitting around in the environment, then that of course would be preferable; rubble, low-walls, boulders, shrubs, trees, that kind of thing.
So that's why I think there actually isn't a huge problem with verisimilitude with this particular issue. (Just as a side note, the combat rules see area attacks as a single burst, given the "origin square" and how the cover rules work.)
As for your house-rule, besides what Starfox mentioned, it would cause a problem for attacks that also cause Zones. They would get a free action to shift out of a zone that would have, without this rule, been able to damage them on the start of their turns or whatever the case may be.