Our Suburban has dual mirrors (normal on top and a convex one on the bottom). It is very handy, and does eliminate the blind spot, but I still need to shoulder check.
I can think of one incident that illustrates the point: me in the left lane, the middle lane clear, and a pickup in the right lane. I wanted to move back into the middle lane, and from both the mirrors and situational awareness, I knew there were no cars in the way.
But, when I did a quick shoulder check to make absolutely sure, I saw that the guy in the truck two lanes over was also craning his neck around in his own shoulder check. So, even though everything was clear at the time, the shoulder check showed me that we were both about to move into the same open spot at about the same time. I never would have picked that up from a mirror check alone.
I also had another close call with a bike courier who came straight at me from the side, a little behind, and perpendicularly across three lanes of city traffic. My blind spot was clear, but if it wasn't for a shoulder check, I would have changed lanes right into him. Of course, in that case I wouldn't have felt too bad about it.