I just find that riding outside is so different from a trainer that I don't give any credence to the trainer numbers anymore. Note that I didn't say to the benefits though, just the numbers. On a trainer, my FTP is about 180, not that high from what I've seen others publish, but outside I can easily ride at a much higher power level. 200 watts? An easy spin. 300 watts, now my HR goes up. But here's the thing I still don't get, how is it I can put out more watts on the road using the same power meter for both? And I can understand that the outside riding is less consistent than the trainer, but I just checked average watts over similar timed rides and the outside rides are always a good 20w greater, and these are frequently less grueling and include easy street riding. I'll feel totally beat up on a trainer, and it worked wonders over the winter. My HR also bears this out. I have a higher HR on the trainer for the same watts.
And to all of you above, speed on the trainer is totally meaningless. You are using your wheel (assuming a wheel-on trainer) to move the resistance unit, how fast it spins has no bearing on how much power it takes.