Coming from someone who used to have a rear wheel sensor for speed and cadence, I no longer see a need for that info when on a trainer. The trainer is about intensity and time. Any mph or distance numbers have no relation to actually riding in the great outdoors. As far as cadence, "slow", "fast", "faster" and "really fast" are probably all you need.

If you need new computer and want cadence for riding outside, then by all means look for one. But I wouldn't spend the money for just being on the trainer.