The only brush I would ever use will be in these areas:
wheel wells
engine bay
rims, tires, mud flaps
There's just too much risk of scratching a car and a lot of those brushes look like scrubbing brushes, once its got that type of bristles on it, don't even consider touching a painted exterior surface with it. The microfiber ones I can understand for hard to reach deck lids etc...
The only brush I would honestly buy auto related is the ones for rims that can squeeze and compress in between spokes and betweens rotors and the inner rim.
Other than that, a cheapish paint brush works the part. I still didn't get to do my handle mod and wash the engine with it.