Kickers give some options. As already posted, they save piling up hours on the main, and long periods of low rpm aren't necessarily good hours.
A kicker is your "get me home" motor if something goes awry. A friend with a cabin near ours runs across the SoG from Nanaimo every weekend, whatever the weather. Full floater suits every time, and if it's blowing at all, the kicker is in the water, running and linkage attached before they cast off. Lose main power in a blow and you have only seconds to regain steerage or you're beam-on to the swell. So that kicker should have its own separate fuel supply, and be just as fussy with kicker maintenance as you are with the main.
There are times when you want to go even slower than 2.5 mph, eg, sockeye, or rainbows on the big interior lakes. Or if you're jigging, bumping the boat in and out of reverse to keep lures dropping vertically.
Kicker and main, or twin mains. Size the kicker generously. Yes, 8 hp will be fine on a 17.5 ft hull for most trolling situations with wind and current included. But most outboard makers build their 8 hp and 9.9 on the same block, so get the extra 25% power, it won't weigh any more but it'll get you home quicker if the main schitts the bed.
