There are many reasons that people name a bank to be executor. They may not have any other person to name or they may want to make sure that there wishes are fufilled properly.
Using a bank as an executor does not change a testator's ability to change the will or revoke it at anytime.
In Ohio the executor fees are set by statute so banks are able to collect the same fees as any other executor.
From your questions, I think you pretty much have a good idea of the answers. Banks and trust companies make very good executors for larger estates. They are experienced, understand the law and are neutral. They will implement the will of the testator. Certainly, they need to be compensated for their expertise and time. The large majority have fee schedules.
The testator can always amend the will or revoke the will. Therefore, in the event that the testator changes his/her mind and would like an individual to serve, that is always a possibility.