I think I may be the 'exeption to the rule' here. When I looked at used late model Marlins a little over a year ago I noticed the neagtives outweighed the positives - for MY needs.
The pro's: can use air conditioner anywhere anytime, same for microwave and water heater. Also better resale value.
The con's: no need to run a/c most of the time up here in the northeast when away from dockside power, water heater stays warm for many hours -just warm it up at the dock ahead of time, add roughly $5000 to a used boat for a diesel generator, difficult to work on, one more thing to winterize and maintain, should run it once a week - can't always get to the boat every week, problem if it needs to be replaced or major overhaul. Boat is approximately 400 lbs heavier all the time (guessing here) due to weight of generator, diesel fuel, additional fuel lines, electrical, etc. I am not a big fan of diesel smell and soot which I assume there is some occasionally based on the vents on the sides of the larger Grady's I have seen up close.
Again, my needs are not typical compared to most on this site. For me it made more sense to get a Honda EU2000 and use it on those three to five overnight trips a season. Cost is about $1000 and can be easily put on the boat when needed.