That is all well and good sitting at your computer. But even if you had a pistol strapped to your waist while awaiting the movie...that's a lot to ask. This dude had God knows how long to prepare for the event. You? Zero.
This douche was NOT vulnerable to attack. He went out of his way more so than any other criminal save the North Hollywood shootout in 1997. Not only did this guy throw gas into the theater and have a gas mask, but he had body armor, 2 rifles, and a pistol (not counting the pistol in his car). His AR-15 has a 100 round mag...100 rounds!!!
If I had my MP9 on me would I have taken a shot? I fucking hope so. But seeing all the facts infront of me am I optomistic it would have saved the day? No. I would hope that MAYBE it would be enough to distract the fucker for a couple of seconds...but a head shot in a crowded theater while choking on CS gas or whatever he used? Yea not taking that bet at all.
In sum: Would a CCW in the theater helped? I'd say yes, anything would have helped. Would a CCW in the theater saved the day? I say no, baring amazing luck and shot placement.