Data shared among devices such as in RAID does make the potential for data loss minimal, and with balancing over multiple servers this becomes even less. The idea of backups (as you said) is changing however. More and more, it's not so much that the hard drive crashed and the data is gone, it's a situation of my site was defaced, or hacked or I accidently deleted a file. In a RAID environment this doesn't help the use. Having incrimental backups allows a user to go back in time on their site and retrieve files that they may have deleted or become corrupt (even database's or database tables).
One other item on the backup is that if the hosting company goes out of business, or the data center is hit with a disaster (fire, flood, hurricane) - do you have the ability to restore your site? Most people do not. In having an OFFSITE backup, you're a little safer. Having YOUR OWN backup is even better because then you can restore at a new host if needed.
I've seen hosting companies fold up overnight. It's sad, but they close everything and leave their users stranded. Featureprice (2004 or 2005 I think) closed their doors with over 100,000 sites. Phone disconnected, everything shut down. I can't tell you how many users came to us with no backups