-- Last Updated: Feb-04-13 6:25 PM EST --
If It is hard to do ;) As mentioned, some boats make it really easy, others - not so easy, and in others, unless I have my PFD on, I can't do it (a buoyant paddle also helps).
The cockpit should not be too tight, so you can wiggle around, especially if you do not have great flexibility.
I have 2 sizes of the same white water kayak and I can balance brace in both. But the larger size is much easier, both because of the added boat stability when on edge and because I have more room in the cockpit for my legs (both are otherwise with the same seat and snug at the hips). In one sea kayak I had it was almost impossible for me to balance brace, while in the others - piece of cake.
As for usefulness, I use it as part of rolling practice or just to relax in the water while paddling. At the beginning of paddling in cooler/colder water I often do a balance brace to get wet and accustomed to the temps rather than roll or splash (often I do a high-brace splashing my head in the water then recovery without a roll for the same purpose).
For perfecting it, start with a pair of paddle floats on your paddle and gradually reduce the air in them till you don't need them. Arch your back, stay away from the boat with shoulders flat on the water, and dip your forehead in the water with only your nose/mouth above water. If you can't relax with the upper end of the boat away from you (rather than trying to fall on top of you), then there is a fit/flexibility problem and trying to force a balance brace might not ever work...