You are assuming modern disks that even have SMART. I still have disks that don't. The disk can be fine until suddenly one day: bad block!
But, even with modern disks, do you really think the average consumer has even heard of SMART? Do you think they have any clue that their disk needs to be replaced? Do any distros even enable SMART monitoring* by default? Do you think bad blocks only ever appear gradually?
Unless you have a specific good reason to not have bad block relocation, why would you not want it? You shouldn't need checksums either, but they are a good idea. A modern FS should certainly be able to handle bad blocks, I would find it hard to call it modern of it can't.
* I am kind of curious when distros will become proactive and install all sorts of monitoring/smtp stuff by default?