Every draft review said Demby is quite smart. The article on him says he's an eager learner.
If we know anything about Snead/McVay draft choices "smart" is high on their list.
Now brain power doesn't make up for a lack of ability, but if you want to know why he's on the team, and why he keeps getting his chances, this might be why.
I also suspect that they want to keep Edwards only learning the left side and that Evans isn't ready.
