Only speaking for myself, of course... but for a few reasons, the 9.9 is an option I could not live without.
I've done much boating, and have had my share of issues with either engines or props, and was always very glad I had another screw on which to get home! My last 2 boats were twin screw, so this (R-23-OB) is my first single in some time, so maybe that has a little to do with it 😉
Trolling is one of the reasons I got the 9.9 also... mainly because I didn't want to put tons of hours on the main OB having it just "idle" for hours on end. I'd rather put many hours on a smaller motor, which is cheaper to replace/repair in the future.
On our R-23, the 9.9 does a fair job of motoring us in many conditions. I've tested it in different sea states and wind conditions, and am confident that it will keep us out of trouble and/or get us home. Ours has the Panther remote, which makes it nice to be able to sit at the helm and steer. No throttle control with the Panther, but I haven't found that to be an issue as of yet.