Apparently they think the Gas turbine is not producing enough electrical power via generators that the ship requires at certain times.
No, the gas turbines are producing plenty of power. The problem is that gas turbines lose efficiency very quickly when they operate at less than full power. When these ships are in port, they often have to run a generator to supply so-called "base" electrical loads -- room lighting, services such as the laundry, the galley, the distilling plant, and the sewage treatment system, etc. -- but that's not sufficient to load a generator completely. The diesel generator will supply these "base" loads a lot more efficiently than a gas turbine generator. With the escalated fuel prices, management determined that the savings in fuel would buy the diesel generators and pay for their installation a very short time. Thus, it makes sense, from a business perspective, to install them as soon as possible.
The pods run on electric power and that is the reason they are so smooth without vibration.
It's true that pods run on electric power, but steam turbines would be just as smooth. In fact, the electric motors are so smooth only because they use a three phase electrical supply. A single phase motor, like a larger version of the standard motors in most home appliances, would vibrate very noticeably.
There must be a defect in the design of the bearing cooling process causing the frequent failure on mostly the Infinity I think.
Emergency repairs of bearings in the Azipod units have been pretty much an annual event on all four ships of the Millennium class. The most common failure has been the main thrust bearing, though other bearings have failed on a few occasions.
The biggest problem with these systems is not so much that bearings have occasionally failed, but rather that the Azipod configuration puts these bearings in external pods where they are accessible only when the ship is in drydock. A conventional shaft would put the main thrust bearings and the propulsion motors inside the hull, where the crew would be able to repair them while the ship is underway.