Side projects are good for you? I think it depends.
Enterpreneurial types in most cases can't specialize (they get bored and demotivated quickly while doing it) but trive while stiching things together and see them work / produce results. This is their motivation and if you are such person and do not get enough of playground at day job you have 2 options, quit or kick off some side projects.
I also admire people who can specialize. Lets call them real engineer types. Only because of them we have all technology things running day to day. But I guess side projects are not such necessity for these guys.
In both cases I was talking about people in software dev space.