Imagine you have weekly backups , but only for the past 26 weeks because of disk space. A file gets corrupted, but you only view it a year afterwards. You effectively have no way of recovering it.
Knowing something is wrong can be useful (though being able to fix it is even more useful).
Needless to say I no longer keep anything of consequence on non-checksummed filesystems now.
I've never had a single byte of corruption. Not one, not ever. The data is checksummed about once a month.
26 is a very high number for a deterministic system compared to zero.
My recentish MBP purchase (2011) resulted in a single corrupt file copying 1/3 of that volume onto the machine over the LAN. this was 10.7 at the time. That scared me a little.
The Linux kit I operate has had no discernable data corruption either in the last 12 years.
I know this is an anecdote but over time that's a huge cumulative error.
I've been using Macs for 5 years now. I've stored time machine backups spanning two drives and OS releases from snow leopard to mavericks. (Each drive was used for about 2.5 years, and the OSes were installed soon after they became available.)
Every 9-15 months, I ask Disk Utility to repair the Time Machine volume. The first time, I lost the entire TM directory and had to format the drive and start over. The second time, it crashed Disk Utility because there were so many errors. The third time, I lost the older ~70% of my backups. The most recent time, the operating system refused to even recognize the disk as an HFS+ volume!
Each time, I had the opportunity to format the disk in question and check it for hardware issues. They all passed badblocks and SMART with flying colors. In addition, the volumes were unmounted properly the vast majority of the time (a few instances of human error and power outage)
tldr: HFS+ loses your data catastrophically even if you use it properly.
By the same token, I've experienced much more corruption on my ZFS (~200TB) and btrfs (58TB) arrays over the last few years than I have on HFS for a decade.
I suspect it's because Time Machine works by creating tons and tons of symlinks, which the HFS+ driver does not like. Regardless, it shouldn't be possible for a userspace application to corrupt the hard drive by creating a lot of links and folders.
A modern computer does more operations in a single second, than a small country can buy lottery tickets in a lifetime.
Many wildly unlikely things become quite likely with computers.
Checksums wouldn't have fixed this, they'd only alert the user to the fact the damage had already been done, which is exactly what the decompressor did in its own special way.
As another comment points out, error correcting codes are the way to handle this, and its already done in hardware, and probably too expensive to do in software in the general case