Voiding warranties is an old myth. It's not true. All trainers do is clamp on to the outside of the skewer. You put much more stress on a frame climbing a steep hill.
The problem with spin bikes is they don't match your road bike in terms of fit. Lots of people get injures putting in heavy miles on spin and stationary bikes. If you spend time indoors it's best to use a trainer or get a high end stationary bike that you can replicate your road fit on.
If you don't ride trainers a lot, just keep the regular tire on all the time. Trainers do wear them out but not as much as many think