I have a polar on my bike and use a trainer where the rear wheel is held by the quickclip to the trainer.
The speed sensor can't be mounted on the rear wheel because it's too far from the computer. I tought it would bother me b ut I realized that I don't care about speed for training.
This means that I get bpm and cadence. It's not particularly easy to move from one bike to another. If I had two bikes I would either get more sensors or just live with no speed and cadence on one of the bikes.
Also you could use a GPS for speed. I intend to use my GPS this summer to validate the speed on my polar. I probably won't need the GPS after that unless I go for a really long ride and want to keep a log of my speed and path.