Had one on a older bass boat. If your good at switching often the may extend your day but once your trolling batteries are dead you spend a lot of time driving around to get any charge in them. Newer motors may put out a little more current now days not sure. The other neg is if you forget to turn it back to your starting battery you can kill that one to.
Alternator output of course varies by engine size but if you've got a 50HP or larger motor you're putting out ~20Amps. 115 to 150HP is 30-50amps, and the larger 6cyl outboards are 50-70 amps.
I don't know how much of an advantage this is during a day of fishing unless you're doing a lot of running somewhere between cruising and WOT. Where I think this is an advantage is if you've got a bit of a run in to the dock at the end of the day you're getting a big start on your overnight charging.
This could also be a big help for someone trying to limp by the last few weekends of the summer/season on batteries in need of replacement.