They are expensive. But without them, i find my head slumping forward, and pretty soon, I'll have upper back problems and I'll end up looking like a hunchback. I can already feel the pain. The problem with me is that I'm somewhat tall and lanky. I have long legs, an average torso, and long arms. So the correct typing position for me is having a low desk so my arms can be relaxed, but that interferes with my long legs, so it has to be just high enough to let my legs underneath the desk. But if I do that, the monitors are WAY too low, and I'm looking down. So, that's why I need monitor arms to raise it up to my eye level.
I never used to care about ergo, but after getting the monitor arms, I've seen how it makes a lot of sense. But the real culprit is my crappy mattress. before that, I never ever had pain in my back.