I have no problem with Holder being asked to provide these documents. I do have a problem with him failing to produce them. If it was true that someone screwed the pooch and funneled weapons into criminal hands then we ought to know. It could have been a matter of unintended consequences, but that does provide sufficient grounds for refusal. I don't know what happened, and now it appears we never will. Holder is off the hook and the mystery remains. Invoking this has been seen in the past as an admittance of guilt by many, but we'll see.
BTW, I didn't approve of how GWB used this neat deux ex machina to cover his butt either.
What remains to be seen is how things progress. Exec. privilege is not unlimited but it is not easily circumvented. If there is sufficient evidence of a conspiratorial nature it can be voided. That's not likely.