What khuon said. You can't coast on a trainer. No tailwinds on a trainer. On the other hand, you can dial in just the amount of resistance (or lack of it) you want.

The dread of having to spend time on the trainer, coupled with all the rain we've been getting this year, has made me a convert. Put on your garbage kit and ride in the rain. Rinse off the bike afterwards, clean and lube your chain more often. I'll catch up with my trainer hours this winter.