I think everyone should be using passphrases, since they are the best way to provide plausible deniability to your wallets, and also provide extra security should an attacker discover your seed phrase back up. However, they do absolutely nothing to prevent someone from brute forcing or stumbling across one of your private keys (which is already so rare as to essentially be impossible before the the death of the sun).
Nicely said, and I think it sums up what matters in all doubts about how safe the seed is and the way it is generated. I think that these things are not completely clear to a large number of users, and they are very important if someone has doubts as an OP. When I made my first wallet I didn't have any doubts like this because MultiBit didn't have a seed, but later I had the same doubts as an OP until I learned that seed is just a simple presentation of something much more complicated and not as simple as it seems at first glance.
However, it is quite logical that people feel safer if the seed has 24 words and a passphrase, and if it helps a person sleep more peacefully, it is good that such things exist
