I am often asked that question and the answer is not simple. Obviously, everyone rides a bike for different reasons and therein lies the rub. If you are a budding racer or someone who wants to learn to properly ride in a group then my answer would be a resounding 'yes' for my sake as much as yours. If you are looking to improve your basic performance on the bike, then you have a lot of options. I guess what I am saying is that when it comes to learning the basic bike riding skills, you should definitely seek out a coach or instructor. If you want to go faster on the bike, you don't necessarily need a coach.
Bike riding skills are not learned from a book. Period. There is just too much going on in a group and too many potential scenarios that written text just isn't going to be able to address them all. My advice is to find a local racing club or skill training session in your area and learn from those who have the experience and knowledge. A good skills course should teach you proper position on the bike so that you will ride a straight line (bend those elbows). Also, learning how to ride a wheel in all situations, single paceline, double paceline, double rotating paceline and echelon is a good thing for everyone.
I am still amazed at the number of people I see out riding who have their hands resting on top of the bars and not using their thumbs or fingers to provide some grasp. This is 'safe riding skill #1' and if this isn't covered in your course, find someone else to teach you the necessary skills.
When it comes to performance coaching there are several possibilities. You can coach yourself which is like a doctor treating himself/herself. To be sure, some of my friends who have been riding forever and racing at a high level do know what their body needs and a coach is probably not going to provide significant improvement. But, the overwhelming majority of riders out there will benefit from some structured form of riding. However, it is much like the old psychiatrist's adage, a patient will only change if they want to change. So, if you really want to improve and get some form of direction, you have to want to follow that direction.
Many riders, especially the more internally disciplined ones, will benefit from a written training program much like those offered by Active.com when you sign up for an event. Much like putting together a model airplane, if you can follow instructions, then this may be the ticket for you. Other riders need more interaction and for them a personal coach is a better match. Being able to discuss how you feel, how your workouts are progressing and what your goals should be on a periodic basis are some of the benefits of having a coach. I think you get the best out of a coaching relationship if you are entirely truthful about your riding and fitness. Holding anything back keeps the coach from properly doing their job and wastes your time and money.
Whatever you choose, the ultimate goal is to have fun on the bike. Please don't lose sight of that because then you are cheating yourself out of some great experiences. Cycling should be a lifelong passion.
Bruce