It would be very difficult, in fact, to use OS X without making use of Keychain, whether you are aware of it or not. Keychain is generally completely transparent to the user but essential to the user's computing experience.
I'll have to disagree here. We see people almost once a week that have a locked keychain that won't unlock. This results from them using their restore disk to reset their password, usually because they can't run software updates because they forgot their password. (auto login is not a good idea imho, for this reason)
The boot disk resets their password, but does not delete nor disable their keychain, so it remains their default keychain, with their old (unknown) password, and does not unlock on login.
Each time they try to do something that can use data in the keychain (like browse to certain websites with forms to be autofilled) or check/send mail, the system sees the entry it needs in the keychain but cannot get the data out, and prompts for the keychain password. Users tend to be very tolerant of clicking cancel all the time before they finally bring it in for us to fix.
Besides the annoying constant popups asking for the keychain password, the user then has to input their email password when receiving (and sometimes when sending) mail, and none of their forms on the web pages autofill. (there are many other minor things that won't work also) So it's quite possible for a user to get by without access to their keychain, they do it all the time.
Irony of this typical mess is it's usually a call to Apple that results in their using their restore disks. Why on earth Apple doesn't tell them to trash their keychain when walking through this I don't know. Then again why the password reset app doesn't manage this for you is also a mystery. Apple going to signed updates to avoid users needing to type their admin password to install software updates seems like a move in the wrong direction.
Something just occurred to me - if the master password is set, and the master password is used to reset a user's password, I know it will fix the filevault key if the account is vaulted - but does it also fix the keychain?