The point is, you're doing nothing but a kludge just to make your fireballs rounder in, what is essence, an abstraction of things. Are squares mathematically elegant for the 45' angles? No. But neither are hexes! Both are kludges for the natural world, but only one is a kludge for artificial constructs.
Specifically SIZED and ORIENTED artificial constructs. Squares are still a kludge for a wide variety of artifical constructs.
And, it's not just to make fireballs rounder. That's a pleasant side effect.
It's to make diagonal movement cleaner. Creatures do not fly across the room, just because they move diagonally. And creatures need extra movement to try to avoid the Defenders that 4E drops on the floor.
It's to allow weird shaped rooms of any size and any orientation next to each other. The DM can have diamonded shaped rooms next to square rooms next to circular rooms, in any orientation and it all just works.
It's to eliminate weird flanking problems like the diagonal flank versus the horizontal flank shift opportunity attack example that I mentioned earlier.
This is also why I mentioned that offset squares are better than even hexes or squares. They have many of the advantages of both hexes and squares and few of the disadvantages.