In a major encounter the pc's might sometimes field 100+ attacks.


If someone has been removed from the thread, it is in bad taste to make a post they can't respond to. It is in exceptionally bad taste to make that post full of snide jabs that are meant to bait another fight. Stop doing that, please.
Now, let me pose a question to you: Assuming that Second Chance is reserved for critical hits until some time late in the encounter, how does this change how you evaluate it, mathematically?
To those GMs that don't tell what AC the attack hit (or at least that it was a close one), or won't even tell what defense is being attacked, please do the gaming world a favor and stop. It does not contribute to the fun or mystery of the game-- it is just frustrating.
For some reason, at cons I get stuck with GMs who like to do stuff like this, and it turns something that was supposed to be fun into an adversarial thing. As if the GM were trying to 'get" the players. It is not worth the frustration on the part of the players. I have learned to just get up from the table and leave in situations like this (because it always gets worse in my experience with this style GM).
For the benefit of those who haven't heard the podcasts - he told the players what the attack roll was, and asked if the attack hit their AC (or whatever other defense was being attacked).
So, we solve for a range of N hits and how much damage is saved on average if the PC uses Second Chance or Shield on the nth hit (in the case of Shield, nth or earlier hit since Shield is used on the first hit it can be used on):
N 2ndCh Shield
1 52.5% 30%
2 57.0% 51%
3 61.1% 65.7%
4 64.7% 76%
5 68.0% 83.2%
This calculation seems reasonable. If you consider a distribution of the frequency of successful hits for encounters as something like:
1: 0.15
2: 0.25
3: 0.3
4: 0.2
5: 0.1
Then you can compute an expected value for each of the powers. The above distribution comes out to: Second Chance: 0.602. Shield: 0.605. Of course, this is somewhat too favorable to Second Chance, because, as you indicate, nowhere does the player have to consider that saving Second Chance might mean it never gets used. Since you’ve assumed the number of successful attacks to be fixed, you can’t quantify this tradeoff.
In practice you can do more than save Second Chance for a critical or Fort/Will status effect as your point 4 says—you can save it for a hit from a Brute with higher damage/lower attack bonus, be more likely to use it against a single AC/Reflex status effect, and so on.
Turning to a different issue, Shield of Faith is a hard power to compare to Shield. The reason is that Shield of Faith is a daily, and requires a Standard Action. So at a minimum, making a comparison requires an assumption about the number of encounters per day, and the value of a lost standard action compared to an immediate action (one in the first round of combat, if you want to get the maximum use out of Shield of Faith), plus an assumption about comparing the utility of blocking a lot of hits in one encounter against the whole party vs. blocking fewer hits and only against the Wizard in each encounter.
Here’s one set of (very simple) assumptions you could make:
We’re only concerned with the average number of hits blocked per day. There are 4 encounters per day.
Suppose Shield blocks an average of 0.55 attacks per combat (n=5, 70% of attacks targeting AC/Reflex, 0.2 extra attacks for which Shield will apply each time it’s used gets you this result). So Shield blocks 2.2 hits/day.
Shield of Faith is useful in one encounter. In the encounter when you use it, the party faces, say, 28 attacks against AC. Shield of Faith blocks 0.1 hits for each attack against AC. So Shield of Faith blocks 2.8 hits a day. Using up a standard action on the first round of combat as opposed to an immediate action when you end up using Shield is worth, say, 0.6 hits (wizards really value actions at the start of combat, but that’s less true of clerics; if a wizard had Shield of Faith I’d definitely pick a higher value here). So Shield of Faith gives a net 2.2 hits a day, the same as Shield.
Which power is better until this metric is going to depend heavily on the number of encounters you assume per day. While more attacks per day means that the number of attacks you should expect in the one encounter you use Shield of Faith goes up somewhat (since you will use it at an opportune moment), Shield’s value improves linearly with the number of encounters per day, which will be a significantly faster rate of increase.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.