Yep, trainer workouts and road workouts differ just enough to present new physical challenges. I encountered that last year when I was out of commission for a few months after being hit by a car. I mostly used the indoor trainer. While it maintained my cardio fitness, it took awhile to adapt to getting back on the road.

With the indoor trainer I never coasted. In the real world we coast a lot, if only a moment here and there. But those brief respites may aid circulation and recovery just enough to minimize injuries.