Great post rookie...Thanks to my environment (Alaska), my available time (3 kids and one on the way), and my resistance thus far to delve into fatbiking, I spend 6 months out of the year exclusively on the trainer. I literally couldn't do it without my coach. Without her planning my workouts and keeping me on track, I would roll into next season fat and out of shape, getting crushed by those that did train, spent their winter on the bike, or cross-country skied (I alpine race). Since I recognize that everything in my life works better when I'm in some sort of shape, I consider a coach an investment in more than just how fast I can ride around in a circle or climb a hill. YMMV.
I usually take a couple months off every year to just ride and do the odd 'cross race- basically to prevent burnout. A couple weeks before I start up with the coach again, I try to get used to a more regimented program on my own. It's plainly obvious that I am not nearly as skilled at planning a workout progression as she is. Where her patterns on the Performance Management chart have the ATL/TSB tightly managed and the CTL on a steady upward pattern throughout the trainer season, my efforts are all over the place. I just don't have the patience to learn how to manage them that well.
I also find that having a coach motivates me because I just don't want to let them down. Week in and week out she's there monitoring everything, so if I slack off she'll notice it. She writes the program, and I do my best to live up to it. That's the bargain. The payoff comes when I hit the road for the first time in the spring and notice just how much stronger I am. That strength allows me to think more in a race instead of just mindlessly hanging on, which greatly increases my chances of doing anything special.
That's just me, though.