There are two things going on here. You are correct that "90% of sailors with a wind vane use it to steer the boat 95% of the time." That assumes of course that the sailor in question has learned how to balance his sails, etc.
You do *not* want to use the Monitor when under power. Scanmar (the mfr) specifically recommends against having the servo-pendulum in the water when powering, because the prop wash will cause vibration and wear on the Monitor components.
Under sail, however, there are some points of sail (e.g., broad reach) where the apparent wind direction varies wildly and the Monitor (or other windvane) will have trouble holding a decent course. In these conditions, a stub-vane with a tillerpilot on the Monitor will do a better job than the windvane.
As a footnote, we use our little TP10 Tillerpilot as an autopilot for our Valiant 42 when powering in calms, by hooking the TP10 drive to a little bracket I made for our Edson wheel. It does a fine job in calms, steering us arrow-straight. You can see the hookup at our "projects" page
(scroll down to the "Cockpit and Deck" section). If it starts getting overpowered, that just means that the wind has come up and we ought to start sailing.
Hope this helps.