The main wear will be on your tyre that is pressing against the resistance. So put an old tyre on there... when we had a trainer we put tyres that had cuts in which were no good on the road anymore - max use for your money.

We have a spin bike now as we didn't like putting road bikes (even old ones) on the trainer.

The LBS guy is spot-on, bikes are made to move and sway as we pedal and to respond to our weight shifts. When a bike is fixed solid on the trainer it stays upright and immovable when you (for example) apply weight on a down-stroke of the pedal. On the road the bike would move a little in empathy to your movement.

Never, ever put a bike you love on your trainer if you have the choice.

Salsa has also hit the nail on the head with the convenience aspect - which is one reason why we chose the spin bike. Taking our bikes (there were at that time 4 of us using the trainer) on and off the trainer was such a pain, and during wet weeks we didn't get the bikes in as often cause not only might they be muddy from road rides, but they would also get wet bringing them in from the shed to the house.