Trainers have + and -

+
Can't coast. If you ever get a chance to see a power meter graph/file of a ride outside, you'll see that there is a LOT of coasting. Therefore there's some saying that an hour on the trainer is like an hour and a half on the road. Something like that. I don't think that's quite accurate, but you'll find yourself pedaling more consistently on the road. I wouldn't say you get "extra training" benefits as far as endurance etc. You can just not coast outside.

-
Can't rock bike. It's hard to stand on the trainer - you'll end up tilting left-right backwards relative to pedal stroke. So you get out there and suddenly you're using muscles you haven't worked. So you get sore etc.

Basically using an indoor trainer gets me to about 70-80% of my overall fitness. On trainer rides alone, over the course of 6 months or so, I can train for a week to three weeks of 4-6 hour rides outside, on terrain that's much more challenging than anything around where I live.

However, I use those outside rides to hone my form. The bike rocking, etc.

cdr