PWM switching in BLDC motors: ratio between switching losses versus conductive
You are absolutely correct, motor speed can be controlled without the use of PWM by changing the voltage.  There are a couple of considerations to this approach.

Brushless motors are not all wound for sinusoidal drive.  You would want a motor with a more trapezoidal back EMF for this approach.  You mention two functions of PWM in your post.  One is obviously speed control, and the second is to match the back EMF of the motor to the drive waveform.  When you go without PWM, the drive mismatch from the EMF means that the instantaneous current requirement will change ("current ripple") and so will the instantaneous torque.  If you have a large inertial load, you will have a lot of current ripple as the motor goes from a position where there is high torque (the back EMF voltage is low with respect to the drive) to a a position where there is less torque, but your inertia will keep the speed relatively constant.  If you don't have a lot of inertia you may actually have some "cogginess" in your output speed.

Your real question is then concerning your switching losses.  If you have a driver with a lot of stray inductance and are switching at high PWM rate, your losses can be reduced by your approach.  We run downhole motors in this manner.  You will still need PWM to keep your current from spiking during start-up.  Get the best motor wind you can, but you will have to put up with current ripple of 15-20%.