Diesel engines with conventional drives still require cooler service, so the money would be spent regardless.
I have run dozens of IPS boats and hundreds of conventional inboards. IPS is definitely more fuel efficient than a conventional drive. Is it better? The answer to that is "It depends". It depends on what the owner expects, how the owner uses the boat and if the owner enjoys the quieter more fuel efficient ride. The IPS drives have to be maintained, but so do transmissions, cutlass bearings and steering systems on conventional inboard drives. The IPS drives combine the transmission, running gear, steering and exhaust into one unit.
In my experience, IPS is between 12% and 17% more fuel efficient when comparing a 40' diesel vessel. I have never run a gas IPS vessel, so I cannot comment on that. The IPS is quieter nearly vibration free. I think it's awesome running a 40' express yacht at 29 knots and getting nearly a mile to the gallon. The straight inboard counterparts generally get between .78 and .85 mpg.
So are Pod dive vessels all they are cracked up to be? The answer is "That depends".
I like IPS for cruising vessels where comfort is the name of the game. The Sovrans are great! Fun to run and easy to dock! For fishing, I much prefer a conventional inboard with big props for positive commands when backing down on a fish. I also do not like the exhaust being mixed into water when fishing. I want to see as much as possible when fishing including into the water behind the boat.
Pods also have their limitations. Weight, beam and load are limiting factors when it comes to pod drive boats. Pods are great for relatively light low resistance boats (express boats). Heavy boat designs (Motor yachts and Sport fish Convertibles with towers) generally achieve better speed with conventional inboard gear with large props and large gear reductions in the transmissions.
Any questions?