I just pulled my fixed ropes from both sides of Mikey's first pitch rope last week. I tied off the core-shot on the first pitch rope so it can be jugged easier but that probably doesn't matter now.
My partner had a helmet in his pack, so we were not exactly without. He usually leads with one. I don't use one, except if it's required for teaching a class.
Helmets may be good if you hit your head during a leader fall. I've never done that.
If I think rocks are likely to fall on me while doing a climb, I'd choose to not do that climb. Example: that thread I created a couple of years ago: "Temple Crag: Too loose? (Accident list)"
And when belaying the leader, I try to locate the belay in a place which has some protection from rocks pulled off by the leader (either off to the side or under a roof).
One time my partner was leading and knocked off a big flake that was headed straight for me. Fortunately I had a long tether (rope to the anchor), so I just swung to one side and it missed me. A helmet would not have protected me.
On 7/24, I was actually hit in the head by a rock. It was my fault for fiddling around at the base when my partner was directly above. My head bled a bit. No big deal. A helmet would have avoided the bleeding, if I had one and was still wearing it on the ground. But I should not have been doing what I was doing.
Here is additional information on the recent rockfalls from Middle Cathedral Rock.
First, all of the recent rockfalls appear to have originated from the same general area on the upper North Face of Middle Cathedral. I've marked the area on this photo with a circle:
Recent rockfall source area on Middle Cathedral Rock, marked by the circle.
The next two photos show that circled area zoomed in. The first photo was taken immediately after the rockfall on 5 July 2011, and the second after the rockfall on 1 September 2011. Note that there appear to be at least two distinct areas that failed (shown by the light colored scars in the second photo). I believe the upper one was the site of the 12 July failure, and the lower one with the prominent water stain through it was from the 1 September failure. I cannot tell yet whether these two were somehow related through stress redistribution, or whether they coincidentally occurred in such close proximity - I think the former case is the more likely scenario.
Middle Cathedral Rock rockfall source area after the 5 July 2011 rockfall.
Middle Cathedral Rock rockfall source area after the 1 September 2011 rockfall.
You can see from the photos that the failed slabs were relatively thin exfoliation sheets, probably only a few feet thick. These thin slabs broke up substantially during the fall, producing the dust clouds. I think this also explains why the impact zone at the base is not nearly as devastated as other larger rockfalls have been. Clint's photos above show the detail of this area really well, and this photo shows the base as seen from the North Buttress Route on 25 August 2011 (we did consider the potential rockfall risk before getting on this route, but decided to accept it given the route's position east of the fall path). Note that the area of fresh talus and damaged vegetation is relatively thin.
Base of Middle Cathedral as seen from the North Buttress Route. Photo by Josh Helling.
As a final note, it might be helpful to consider the Rhombus Wall rockfalls, which were similar in nature and occurred sporadically over a 1.2 year time period.
Hope this is helpful in deciding when/whether to venture up there again. Regardless, it couldn't hurt to wear a helmet.