Why not ? Commmon stepper motors have 1,8 degree steps, while a typical commutator DC motor has only 3 or 5 commutator segments being discretely energised per revolution, is that any smoother ???? It all depends on the drive electronics, with the right timing a 'stepper motor' will be far smoother than a commutator motor.
You clearly do not understand the construction of these types of motor. There is really f*ck-all difference between what is called a 'stepper motor' and what is called a 'brushless DC motor'. Mechanically, they are virtually identical. The differences lie mostly in the drive electronics for the application, and the way the stator coils are terminated. The 'brushless DC motor' usually (but not always) has Hall-effect devices mounted to sense the rotor position and control the phase of the (typically)
3-phase coil drivers, while 'Stepper motors' usually don't have rotor sensors.Have a look at the May 2008 issue of Silicon Chip, page 16. Leo Simpson argues that 'In reality, there is no such thing as a brushless DC motor', and I totally agree that the terminology is misleading.