PP,
Thanks for a reasoned response - unusual in these parts.
Banknotes can be faked, so pretty much anything else can be - if you follow my sentiment. So yes, what I am saying is that ID check is one of a number of gateing factors. None are watertight (e.g. for pax, only 1 in x is body searched) but that does not mean they are mutually exclusive. It's not binary - less than 100% useful doesn't mean 0% useful.
So, you search 1 in x knowing you didn't check them all, you check every ID, but you know some (small number) could be fake, etc etc. Combine a variety of checks and their respective odds and the chances of a rogue getting through kitted up is reduced to long odds (although not zero).
If on the other hand you say - all IDs must be real, so we'll skip ID holders... all passengers who are old ladies in wheelchairs... we'll skip those too... now you start to open up the odds.
Basically the more stereotypes (ID holders, old people, children, the disabled) are seen as officially "good" the more these identities become obvious disguises.
I hope some of this makes sense - my security knowledge isn't from aviation, but it's just applying basic principals. Find the vulnerabilities as if you were the attacker.