Interesting. He does mention it is his opinion and that some good players do hold with the index finger curled.

Where I am seeing the closed grip is on the context of metal and speed riffs. I wonder if he had that in his frame of context? I might post this in one of the instructor forums, since we're not getting much input here.

Just to be clear: I am not trying to say this is a good thing to do or not. I am trying to find this out.

[Edit:] I posed the question to Lawrence Katz in his forum and got a very fast reply:
http://www.guitartricks.com/forum/showthread.php?t=31591