This is true. I would certainly be making a lot more money these days if I had specialized. And apart from that, when you know a bit about a lot of things, everyone expects you to do everything. When you are specialized people let you do the thing you are good at and don't distract you so much with stuff you are not supposed to be working on. Mostly because they don't want to pay you to do those other things.
Really good point man. The willing horse gets all the work and often you end up doing other people's jobs for them too, and can be to your own detriment. Do you work in technology field too?