I find this somewhat hypocritical...depending on how you do it.
Where do you house the HDDs? If it's inside the server, then they can be accessed, then it's not a true backup. If it's elsewhere (in the DC), then you have to pay for extra colo space and power. If it's at your house, then you have to pay for massive amounts of bandwidth used frequently in backups.
In addition, any server HD has to be made especially for 24x7 use and is quite expensive. You don't use your typical consumer level SATA HDs in servers (unless you are very silly).
I didn't mean to imply that raid = backups, I just thought it was what was meant.
At the end of the day, backups = higher cost = higher prices = less customers.
It is best to make sure you don't have any data loss. Backups are still important, but IMO it should be the client's responsibility to do backups and not the host's.
Of course again, this is to do with price.