They only go up to around 1200W, and those cost $200-300. From what I've seen, when using the immersion water-heater elements, you need at least a couple thousand watts to comfortably boil a batch - and immersion-type elements would inherently be more efficient because nearly all the heat they generate would be going into the liquid, whereas these band heaters would lose a lot to ambient unless seriously insulated. You'd probably need 2 or 3 of them, plus a whole bunch of insulation, resulting in a total cost that could get dangerously close to 4 figures, and probably not work any better than the common existing methods.
So, I don't think they'd be a good choice, especially for a boil kettle. If cost were no object, they might be sort of nice for maintaining mash temps, though the thermostat would likely need to be replaced with something more accurate...