They obviously do, because you cannot have both, at the same time, on the same key, moving simultaneously in the same transactions (because many people create obstacles called "replay protection", while many networks would benefit, if every transaction would be 100% replayed). So, it creates an unnecessary competition, where in theory, you have 21 million coins limit, but in practice, as long as you can flood the market with a lot of new monetary systems, people will stop using the original network. And the supply of new altcoins is unlimited: you can create as many new altcoins, as you want, abusing the maximum supply.

That's the problem: if things would be signature-based, then you would know exactly, what is signed, and which order is signed. But if signatures are ignored, and things just fly in a way, as they are specified in a given transaction, then they can be re-ordered in many cases.

For example: some wallets sort transaction outputs in ascending/descending order, or shuffle them specifically, for example by sorting them by address types. Then, you can trace someone, if you know, which client is used by that person. And then, if sats are flying in unsigned way, you are forced to give up your privacy, to put a given satoshi in a specific place.

Of course it does. See: https://en.wikipedia.org/wiki/Interesting_number_paradox