Maybe one difference is between people who are looking to use a second engine as a kicker vs. people who are looking to use it as a potential "get out of the road" motor if the main engine conks out for some reason. I'm thinking of "kicker" as an engine that someone plans to use as a regular propulsion device (say, a person who - for one reason or another - doesn't want to use the main engine for the regular trolling they do); vs. an engine that is basically a mechanical "backup oar" (that also may do double duty as a dinghy engine).
As you say, different strokes for different folks; I'm just thinking in terms of establishing common concepts. For example, if I were thinking of using my second engine as the type of kicker I described above (and it sounds like this is your use scenario, if I'm reading you correctly), then I would definitely keep my Honda 8D. It's a great engine, has useful features (electric start plus manual start, alternator, tiller), and plenty of power for displacement speeds in varying conditions.
On the other hand, if I'm only looking at it as a "mechanical oar" in times of trouble (of course with regular maintenance/running to be sure it works), then I'll probably go with something like a Honda 2 and save nearly 100# on the transom (plus be able to use it double-duty as a dinghy motor if desired). But as Bob notes, with the latter sort of set-up one is not expecting to run for home at speed and get there on time; but rather to get oneself to a safe place and then deal with the problem/make arrangements/etc.
To my mind, there's ultimately no practical way to satisfy all possible scenarios on boats of our size (or even larger) -- so each person has to weigh the options and choose the method of compromise he or she is comfortable with.
Sunbeam :hot