I'm glad someone brought this up, because I have another thread open that addresses the opposite situation.
On my smaller sites, I backup my own stuff whenever I update the site (and I copy down my db's twice a month).
On the bigger sites I pay someone off-site to do backups once to twice a week. It all depends on the size of the site and the intensity of use on the SQL.
You're also right in saying that hosts should really do some backups themselves, but this is seldom done on anything less than reseller or dedicated packages (or so it seems, comments?)...
~Equinox