I posted about this topic in a previous thread, so I'll copy/paste that here with some additional comments thrown in:
We have a T9.9 kicker on our '21 R27OB, and for two reasons we would not be without one; #1 is that we troll a lot and #2 is that the cost of a kicker is cheap insurance to have another means of propulsion.
With the pattern-shifting feature on the main that Reel Salty Bliss and Nwdiver are referring to above, speed may no longer be an issue, but it was back in 2020 when we could only idle the F300 down to 650 rpm and then increase it in 50 rpm increments with Troll Mode. In some cases, you can't go slow enough with those limitations. We use an iTroll remote throttle on our T9.9 and the throttle resolution is awesome; from the helm we can increase or decrease speed in increments of less than 1 mph. But, lots of owners of FM boats catch plenty of fish by trolling with just a main. I don't know Nwdiver but I can tell you that Mike & Sarah are as fishy as they come and they have tons of experience.
A bigger issue for us is engine hours - we can easily put in 6-10 hours a day trolling. At that rate, I could be hauling and changing the fluids on our F300 every six weeks if the fishing is slow. Back in July during salmon season, we had about 30 hours on our most recent oil change on the F300 and the oil was turning dark. I don't know how many hours were on the T9.9 (both get oil changed at the same time) but I'm guessing it's was north of 60 hours in the same time period and the oil looked like it just came out of the bottle. I service both engines at 100 hours religiously but I would not have any issue going twice that on the T9.9 It's just more efficient to putt around at 2 mph with a kicker.
Maybe something else to consider - we sometimes troll in crazy currents in the Sound that can change directions in very short distances. Couple that with a ton of boats in a small area and you need to be very conscious of where your rod and downrigger lines are, particularly with the factory downrigger pads being more mid-ship. I've twice wrapped the port braided downrigger line in my kicker prop. Both times, the kicker stalled after the prop was sufficiently wrapped up. I maintained course with the main while I raised the kicker, removed the prop, and unwound the line. Put the prop back on and back to fishing. I think that's less likely to happen with the main given that it's centered on the boat, but I'm sure with the torque of six big cylinders vs. two small ones, there could be a lot of carnage if it did. Since that time, I've thought about how I'd do an in-water prop removal on the F300 and fun times would not be ahead. It may be easier on an R25 with the wrap-around swim platform, though.
Regarding redundancy, modern outboards are very reliable. I'd guess that fuel contamination would be the most likely cause of a failure of your main engine. Fluid Motion uses the same fuel tank for both outboards, so your kicker would be affected as well, unless you carry another source of gas. But other issues can happen that aren't related to fuel. We had an incident in which our F300 shut down in Puget Sound and would not restart. That's a helpless feeling, to have no means of control while drifting into a ferry lane. Having a kicker allowed us to regain propulsion and move the boat where we wanted, not where the wind and tide wanted.
I've seen some comments that a kicker would be ineffective in severe weather. We've been on the kicker in 15-20 knot wind and still had control. A big issue with short, steep "square" waves is that the kicker prop comes out of the water briefly when the boat crests the wave. That said, if you told me I was headed out into a gale and that my primary engine would randomly quit at some point, then gave me the option to have a kicker or not, the answer would be easy...I'll take the kicker every time. Anything is better than nothing.
