It would have the following advantages:
1) extreme control in reverse
2) steering control if main steering lost,
3) alternative if Yanmar has mechanical , electrical, cooling problem,
4) alternative if no gas,
5) alternative if dead batteries.
6) extreme control in forward.
7) far cheaper and more versatile than stern thruster.
as I think about it I remember that I once knew a guy with a 50 foot boat who used a small outboard in forward to get into tight marina spaces.
I owned an ODay 25 with a Chrysler Sailor 250 (9.9hp, 250lbs of thrust) for propulsion. I currently have a Mariner 19 with a 3hp equivalent electric outboard. Let me go through your points.
1) An outboard powered sailboat sucks in reverse. Even with the geared down, big prop Chrysler reverse takes time. Usually lots of cavitation in an emergency reverse, instead of stopping you. Same with my electric outboard. Unless you rig a remote throttle/shift, you have to reach the side of the outboard to shift it while it's in the lowered position. Easy to lose sight of everything else while you are fumbling for the gear shift. The electric outboard is much better in that reverse is twisting the tiller handle the other way, but it still has cavitation issues in reverse. When the electric outboard prop does grab in reverse, it tries to tilt itself out of the water. Steering with both the motor tiller and the rudder at the same time isn't going to happen. You either steer with one or the other - I use the rudder. Extreme control in reverse is a laugher.
2) Steering control if main steering is lost. A good point. I had the lower gudgeon break on the ODay 25 20 miles from Bimini in the middle of the night (enroute Bimini from Nassau, transom mounted rudder) Was able to drop anchor in the middle of the nowhere. In the morning, DW suggested steering with the outboard to get to Bimini. I was in my 20s at the time, and reaching over the transom to steer for 4 hours was a back breaker. Not sure I could do it again. But there you are. A valid reason for an outboard auxiliary.
3-5) Yes, you have a separate system for your outboard. But is it any more reliable than your Yanmar? On the Mariner, I had a 5hp Honda which gave starting anxiety, not range anxiety. I could never depend on getting that thing started. I am old and my shoulders are weaker than they once were. I ran it dry every time, used non-ethanol gas, and still could not get it started consistently. Last year, I gave up, sold the Honda, and bought the Epropulsion Spirit electric outboard. Unless you are using the outboard regularly, counting on getting it started is iffy unless it has electric start. The Chrysler did have electric start.
6) Again, you can't steer with both outboard and rudder at the same time. You pick one or the other. I use my tiller, and lock the outboard facing forward. Others steer with their outboards and lock their rudders.
7) Possibly true, but you won't be using the outboard at the same time as the main engine. You just can't coordinate the two well enough.
A suggestion to prove this to yourself. Borrow somebody's outboard - anything small will do. Temporarily mount it in the intended position. Then try to reach your tiller/wheel/engine controls while reaching the outboard controls.
Fred W
Stuart Mariner 19 #4133 SweetP
Yeopim Creek, Albemarle Sound, NC