I imagine you used the brush wheel at a much lower velocity then the dremel is capable of right?
I was thinking about this too (I got the dremel accessory kit). My dremel can go up to like 25k RPM (battery powered).
A bench grinder that a wheel is typically attached to goes to like, 1800 or 3600 RPM). Even an angle grinder is only like 5k.
I imagine running a brush wheel at 30,000 RPM is maybe not the best idea and that the brushes should be used at the lowest speed setting.
I suspect the important factor is the linear speed of the bristles contacting the work.
Suppose a bench grinder is running at 3600 RPM and the brush wheel has a diameter of 4 inches, then the outer edge of the wheel will be traveling at a certain speed, call it V. This speed will determine how effective the brush is.
Now imagine a small brush wheel on a hand tool like a Dremel. Perhaps it has a diameter of 1/2 inch, which is 1/8 of the big wheel. If we want the outer edge of the small wheel to be traveling at the same speed as the big wheel, the small wheel will have to be spinning eight times as fast, or 8 x 3600 = 29,000 RPM. So in fact, running it at 30,000 RPM is not necessarily a bad thing.
Of course, if you want the action of the brush to be less aggressive and more controllable, then it doesn't hurt to slow it down.
The other thing to note, of course, is that if the contact speed is the same on the small wheel and the big wheel, then the wear rate of the bristles will be the same. Since the big wheel has much longer bristles it can absorb much more wear than the small wheel before it starts to get noticed.