It depends on how you define "unnecessary."

I own a Computrainer. It's an expensive piece of equipment, but it also allows allows me to be extremely efficient in my training. I can program ergometer workouts, so that I generate the exact number of watts that I want to for an exact amount of time just as easily as I can program in profiles of races courses that I have planned on doing.

I'd say it's the best single piece of training equipment I've ever bought (and it's going on 7 years old, so it's pretty durable).