Signatures don't do what you think they do
I say this not to engage in broad philosophizing but because I have a very concrete point to make about a very specific thing: signatures. People think of signatures as being a strong form of physical evidence, useful in court proceeding for proving that a particular person really did sign a particular thing. While this belief being widespread does a good job of denying that things they signed are actually their signature, which is a good thing, the claimed difficulty of forging signatures is simply not true. Anyone can practice forging a signature from a few samples for a few hours and be able to do a passable replica. Anyone with decent skill who practices a bit can get quite skilled. And people aren't very consistent about how they sign their own signatures, making even legitimate matches sometimes look fake. Thumbprints would be far better as a piece of evidence.
Despite that, signatures are still very important and good at what they're used for. What is that? It's to make it clear that someone knows when they're entering into a binding agreement. You can't be forced into an effective contract just because you said 'yeah, whatever' when asked if you want to participate, and you can't be forced into a contract by being tricked into signing a document which says something different than what you think it says. The theory of contracts is based on parties mutually agreeing to be contractually bound, and requires they all go through sufficient ceremony that it's clear when a contract has been entered into (sometimes merchants can get into binding contracts much more easily, but that's because they're expected to be more savvy, the law is big on protecting little old ladies from being suckered).
For example, take the use of signatures for receiving packages. There isn't even a contract entered into when a package is signed for, but the reasoning behind it is the same - it's to make clear that the person receiving the package knew they were receiving a package, and not claim later that there was a misunderstanding. To the extent that the signature has any evidenciary power in this case, it's mostly in that people generally by default put down their real name, and since the delivery person generally doesn't even know what the potential name of a recipient might be, it's hard for someone to lie later and claim that no package was delivered at all.
The hoopla around cryptographic signatures is largely misplaced. Having signatures which were on a web page which clearly stated what was being indicated and the signature was done by moving the mouse like a pen in a drawing area would do a much better job of indicating what signatures are supposed to indicate, and probably be much easier to back up in court later.
Now somebody please explain this to Bruce Schneier, because he doesn't get it.