Trains here are bad and the ones to/from Frankfurt are almost always delayed IME but surely you have other options to fall back on for getting there (or to whatever airport) that take less than "several hours".
Good luck with those fallback plans when your train stops in the middle of nowhere for an extended period of time. Or getting a full refund for that taxi you had to take to reach your intended destination.
I don’t understand why someone would take the train instead of his car unless he wanted to take the risk of losing the flight.
For starters, you don't have to worry about getting stuck in traffic, getting in an accident or the car breaking down. You also don't have to worry about parking the car at the airport.
I guess one's viewpoint on the matter depends on the public infrastructure in one's region.