So, I just grabbed some serrated knives I've had over the years; most are folders, most recent is a "dive knife" with a partially serrated edge I used in the FL Keys a couple weeks ago. If I think I need serrated, I prefer a second knife that is fully serrated. Some multitools are perfect as that's the blade that gets the abusive use, saving your plain edge for more appropriate tasks. I'm not a mariner at all, but if you're cutting a lot of thick rope (especially wet nylon), your plain edge is going to dull quickly...do you have time to touch up or sharpen, or do you just want to use that serrated edge to get the job done?
Another reason is defensive. I'm torn on this. My close friend took a Spyderco defensive class decades ago. Serrated edges can be devastating, or they can get "clogged up" with clothing...which is why they always recommended carrying two Delica or two Endura serrated folders. Again, I don't mind carrying a fully serrated knife, but it's always going to be secondary to a plain edge. The original SOG Pentagon kind of gave you a choice of either on the same blade...
ROCK6