Ahh, you've hit upon my favorite of those cases. I see that all the time. I'd say that about 75% of the people that I've observed during my time as a support person do what you're describing. Another thing I've observed is that it seems only 10% of the people I've assisted know how to Alt+Tab.

It saddens me that to these people, tab will always represent an indentation in a document.

But I don't think the issue is that they're welded to the mouse. The problem is that they switch too much. If I'm typing anything longer than four letters, I have both hands on the keyboard. Other wise, it's right hand on mouse, left pinky almost always on Ctrl