plus lets be honest most people probably skip over that step if it's not required. 
Exactly. Just like most people would skip over writing down their seed phrase twice like you suggest. They can't skip a hard coded checksum, however.if it was that simple people wouldn't come on to this forum saying their seed phrase isnt "working".
And if checksums didn't exist, and someone comes saying their seed phrase isn't working, you have no idea if it is a problem with the seed phrase itself or if it is a problem with something they are doing with the seed phrase (passphrase, derivation path, etc.) By having a checksum, you can immediately narrow down the problem.being able to detect up to 4 characters that are in error sounds good but if it can't fix it too then i'm not sure how useful it is.
Extremely useful. You don't want your wallet software accepting an incorrect address and allowing you to sign transactions to that incorrect address. And by showing you have an error, you know you've made a mistake in your process somewhere or have some malware and can re-examine your process before losing your coins to an incorrect address.