Are you saying it's bad that there would be 128 possibilities for the last word? What's the problem with that?
if there's only 128 possibilities for the last word then what's the point of having one since it is easily guessed. easily brute forced.
can't write down 11 words?
I don't see any reason for not writing the 12th word. But if you have written down 11 words and don't have the 12th word for any reason, it can be easily brute-forced and there wouldn't be a big problem.
why not just write down your seed words twice in a row on the same piece of paper. double the security. no checksum needed.
And yes, it is important. If you don't have a checksum and import an incorrect seed phrase, then you have no idea you have imported an incorrect seed phrase. You could spend weeks or months trying to brute force a passphrase which doesn't exist, or searching weird and wonderful derivation paths, or who knows what else, trying to hunt down your wallet. With a checksum, you know immediately one of your words is wrong and can immediately narrow down your search significantly. Not to mention that brute forcing an incorrect seed phrase is also quicker with a checksum since you do not have to derive addresses and check them for balance for all the invalid phrases.
I think the checksum idea is a badly implemented one. Sha256 is good for checksums why? I think bech32 has a more robust checksum thing going on but I found it impossible to find a good explanation of that made much sense.
Also the whole concept of a checksum embedded into your seed phrase is questionable since someone could write down a wrong seed phrase and the software could just correct it for them and they would never even know they were entering something wrong. i guess?
also well i could go on but you get the idea.