Fuzz,
I don't want to interrupt your track here, because I understand and know it is valid. Let me speak a bit to what I have experienced.
I tend to ride more of a style, that is butt back and a slight bend at the waist forward with my arms supporting some of my upper body weight forward. This posture does require some lower back support as I got from the Corbin Dual Tours (I rode three of them over the years) and was very happy with the results.
Further; with this riding style, and lower center of gravity (shoulders down) I was more comfortable with a counter balance riding style in the corners. In other words, if I were leaning the bike down and cornering left, my upper body was leaning right. So all the bend was at my waist, the hips were over to the left and my shoulders and upper torso were right with shoulders perpendicular to the pavement. That is what I got out of the Corbins. This may be very differant that what other folks have experienced.
Now, flash forward to the upright (Touring) riding style. When I got my first Mustang seat, I realized, very quickly, that this was something differant. By that, I mean that the seat, forced me to sit in an upward profile (more forward) than I had not been accustomed to. For me, this was very out of my riding style, as I was not able to counter balance like I had been accustomed to. So I changed my riding style in the corners and found that, when upright, my shoulders were at a higher center of gravity, and this allowed me to lean into the corners with success and comfort I had not found in the past. When cornering left I was now able to lean my upper torso and shoulders left into the turns with comfort.
I have modified the Mustang seat to accommodate me and the 09 SERG.. I like the upright stock posture, but still found it to be a bit too far forward. As such, I have cut the seat back some, horizontally approximately one inch. That allows me to lean, ever so slightly, forward. Guess I have found a happy medium between the two riding styles that I have experimented with.
I believe, that if you prefer and are accustomed to a more upright riding style, then you will need a rider backrest. Yes, I see a mount / dismount problem there as well. Not sure how to accommodate that, but I will likely need to find out.
If you are shorter in structure than 5'10", and ride an 09 SERG, then a Mustang seat like I bought, will have to be cupped out. I took one inch out of the rider's seating height of my stock seat and I am 5'9". That allows me to place both feet, flat on the ground (barely) at stop lights with the bike straight up.
If you have more experience and comfort with a sport bike riding style, then a back rest will do you little good..
For the time, I am seriously considering a back rest...