If you want to win (or do well), in races a coach would help unless your willing to put in the effort of learning and making your own plan. Being busy with work, a coach keeps me focused. I tell him what races I'm riding in this year and he builds the plan accordingly. It certainly helps when you have a fixed amount of time available and need to get the most out of your weekday training rides.
That being said, Pcad's right, having a coach can take the fun out of riding. I never used a iPod while riding until I had to a coach. Once late August hits the weekday Z2 rides were dull as living downtown TO the only pace to ride without Freds, ped and stoplights, is a loop around the EX grounds. It's could be that I was riding the same circuit every weekday that got to me bored...not the riding itself. Aside from the fitness coach in 08, I had a racing coach ( national track champ ), who was great in improving riding style and race tactics. The group rides were pretty kick ass and I learned more in 6 weeks than a lifetime on BF. This coach made riding fun, a fitness coach can make a program where some days are dull. You want to hammer but instead just cruise while the Tri-athletes go past.
My program for 09 starts with training camp in Cuba end of next week in SC in March. It's much easier to get the wife to agree to these trip when labeled as "training".
At the end of the day, if you can, get a coach otherwise read books and make your plan. The book "Base Building for Cyclist" by Thomas Chapple, and "Racing Tactics for Cyclist" by Thomas Prehn which I've found useful. The one by Thomas Chapple can be a little technical if your not already an athlete.