SANs are about:
1) Eliminating "spofs"
2) Getting _high_ uptime.
3) Virtualizing storage.
In a properly designed SAN (and most are, after all they spend $$$$ to achieve this) you will not have to shut down to upgrade stuff.
If you have one cluster, well, maybe DAS is ok. Then you add another, with some more disks, then another with a new OS and some newer disk, and another, with different raid-controllers, then you upgrade and fiddle and after a while you're in a mess...
Performance *shrugs* yes you can get good performance from a SAN, but you can get as good or better performance from lots and lots of disks for less $$$ so that's not a "selling point". The application is ALWAYS the problem, but people don't/can't fix that, second the database/operating system is the problem but they don't understand how to fix that, so finally it always comes down to "throw more hardware at it!!!".
The point is, they really ARE useful, for their purpose (centralizing, securing and making storage "virtual")