In my opinion, someone who does their job well and professionally is just fine to work on side-projects. Ideally, your productivity and focus stays the same as they did before your side-project.
I truly do not think it makes sense to demand every person on a job is passionate about the job they are doing. There have been many times I have had to (even at a job I normally loved) do long, boring, and fruitless work. I did not do them out of my passion for the "Mission to Save the World of Enterprise Banking", but because I am a professional who does what needs to be done and who does it to the best of my abilities.
When I said passion before, I was trying to express two things.
First, I was intending to express the attitudes that cause someone to stay abreast of their field, keeps their own saws sharp, (and ideally helps teach/inspire others to do the same), comes in and works hard, and pushes themselves and those around them to be better. Perhaps the best word for that is just professionalism.
The second concept is that I would rather be around someone who, while acting like a consummate professional, also enjoys at least some part of it. I love the fact that software development has more than I could probably ever learn, and it is a worthy challenge to seek to master its different forms. I have worked with several professionals who really do enjoy parts of the craft that I only barely tolerate, and I enjoy that about them immensely. They open to me a new worlds of appreciation for the craft. Just recently, I had to work on a few hundred legacy bugs for a few weeks in a row. They were unexpected, not caused by me, and deeply tedious. Some took hours to locate the cause. One of my coworkers really came alive doing that. He just loved the hunt and chase, like a detective working out a mystery. While I was mentally moaning and groaning after week two, he was just getting started. We talked about it for a few minutes one day, and I decided to try and see things his way. While it didn't cause me to love bugs like a fine wine, I was able to bring my attitude around from a silent scream up to mildly interested. Maybe the best word for that is having a good attitude? Enjoys some parts more than others, and keeps quiet when they don't enjoy it? I am not sure, but that whole concept I find to be important. The person who reads books about development and enjoys reading them is going to bring a wonderful attitude to the team, and can motivate others to pursue their own interests.
