It does not affect the sound
quality directly, but it does affect the
feel of how you play the instrument. A fully weighted keyboard with a full range of velocities captured and transmitted will improve the authenticity of what is played. On the other hand, with the ability to edit midi after the fact, you can achieve much of this by simply editing the velocities and release times in the PRV. It is more work this way, and will remove you a step from the music playing aspect and push you towards music programming.
Dan