Dan -

Two issues in my mind with a vernier throttle:

1)  I takes a little more effort to push the button in and move the control.  Throttle changes happen so often that pushing the button is too much extra work.  The friction lock holds it in place, yet is easily "disengaged" by pushing harder.  I would also hate to need power fast and not have my thumb on the button.  In this sense, having a friction lock rather than a vernier is a safety thing, at least to me.  The changes to the prop and mixture are far less frequent and have less severe consequences if a change does not happen immediately.

2)  We (at least I) generally "fine tune" prop and mixture and find screwing the knob in for minor adjustments is handy.  Not so with the throttle -- big changes are more typical.

That's my two cents.  I would not have an airplane with a vernier throttle, and I would also not have one without a friction lock on the throttle.  I prefer verniers for prop and mixture but could live with anything that can be locked at its current setting.

- Rob Wolf