Most as you mention will have backups or USB pen drives with copies of the original install files.
It'll only be a very small percentage of users who will need to scrounge around looking for assistance on how to reinstall an operating system they should have had backups for right from the beginning.
Just to expand on SGilbert...
Macs for the last couple of years come with Lion, and now Mountain Lion.
Apple does not provide a method to install those systems from optical drives, so no need to supply optical media to reinstall the system.
And, if the hard drive (or SSD) should fail completely, and you have to replace the boot drive, recent Macs can connect to the internet, and download the system from Apple's servers, if necessary, without any local backup or disk involved. http://support.apple.com/kb/HT4718