The irony of your calling someone else clueless...
Drives do indeed have checksums on their blocks. That does not prevent them from sometimes feeding you back garbage anyway -- see misdirected and phantom reads and writes. Since ZFS uses a self-validating merkle tree, whereas disk checksums live in the same block as the data, ZFS is largely immune to this problem.
If you've worked with disks any length of time, as in actually trying to write a robust filesystem, you'd know that disks sometimes lie. They usually work but every now and then they do the most ridiculous things, due to mechanical, electrical or firmware problems. That's why filesystems like ZFS were created (what, you thought Sun spent man-decades of expert time on it for giggles?). kthreadd is correct.
Please just stay away from storage. The topic is much more complicated than you make it out to be.