Twenty-four-word seeds are undoubtedly more resistant to random attempts of brute forcing, but at the same time, it is sometimes a tough task to back them up properly.
They are just words, I do not think 24 words should be difficult to backup. But in case you have the backups already in different locations, and you want to memorize it, 12 words would easily be memorized than 24 words. But it not a good option to memorize seed phrase, but I understood your point, if provided, it should just be the 4th backup, but not depending on it.
Moreover, the private keys produced from such lengthy seed phrases can't provide more than 128 bits of security, which makes the whole idea of using very long keys less attractive.
That is true.
About brute force. 12 word seed phrase is secure enough, that is what we should just point to.
Also in a 12 or 24 word seed phrase, if only one word is missing, it would take the same time to brute force it. It would take the same time to brute force the same number of missing words while having more missing words, it become more impossible for brute force.
So if anyone thinks he has 24 word seed phrase and attacker got hold of 22 words from it, the time required to brute force the two missing words would be the same time for 2 missing words in a 12 word seed phrase.
As 12 word seed phrase is secure enough just as we see it from Lucius post, the point of going for 24 word seed phrase that has just 128 bits of security is not necessary. 24 word seed phrase makes brute force more impossible, but no significance in reality.