Lately I’ve noticed something curious: While I used to turn to such utilities every few months, I haven’t had to do so in a long time—certainly not in the past couple of years. Anecdotal evidence suggests that I’m not alone in this; disk errors beyond the purview of Disk Utility seem to have declined sharply.
So I think a huge reason why people don't worry about dedicated disk utilities anymore is that backing up has become such a literally thoughtless process, a hard drive crash is no longer the horrifying event it used to be.
I used to swear by DiskWarrior, but it’s now been a long time since I’ve used it. I used to run it periodically to optimize my volume’s catalogs, which really did speed things up, and several times it saved drives that had become completely unusable. Nowadays, I can imagine using it to get at very recently modified files if my drive became inaccessible, but I would no longer plan on resuming use of the repaired drive. Instead, I would erase it (or replace it) and restore from a SuperDuper! clone.
Fragmentation can still be a problem for large files such as an Aperture database or Spotlight index, which can accumulate thousands of fragments. SSDs reduce the effects of fragmentation, but I still find it helpful to defragment certain problem files using iDefrag.
I also continue to find bad blocks on my drives, which can cause hangs and lead to damaged files. Drive Genius’s Scan feature is the best way that I know to detect them.
Lastly, it’s alarming just how many disk problems my customers encounter. SpamSieve now includes various diagnostics to detect and fix incorrect permissions and corrupt files. It also validates parts of its own bundle and advises the user to download a fresh copy if components are missing or damaged. Seeing how often this can happen, it’s a wonder that anything on the Mac works.