I used to have an VERY old mountain bike on my trainer, until it died.
I have my other road bike on it right now. I mostly ride my new bike. So it is really nice to have another bike to have soley on the trainer.
That being said, I have a triple on my trainer and compact double on the bike I ride on the road. So things are a bit different, but I get the miles and work outs done.
If I didn't have this luxery, I would probably do as Becky does.