Have to agree with Dan here. It's a must!!!
A bit further....one of my pet-peeves while driving is when other drivers like to "hang-out" in my blind spot. I check there quite a bit, even when I don't plan to change lanes, so if I see someone there, I'll hit the turn signal to that side. If they're a couteous driver, they tend to back up. But usually, they'll speed up and stake a claim on the spot right next to me...either way works...plus I have the bonus of knowing what kind of driver is near me.
Randy