You must keep your password and other account details secret (other than your Online ID),
and not share them with anyone else, to prevent unauthorised use of your Sony Online Services account. If you share account details with someone who uses your account in breach of these Conditions or we believe that your account has been compromised, your account and/or access to Sony Online Services may be suspended or terminated. We are not liable for any unauthorised use or sharing of your Sony Online Services account.
http://legaldoc.dl.playstation.net/ps3-eula/psn/e/aunz_tosua_en.html
Sony put the feature there so family members (children) can have there PSN on a shared family unit.
Sharing with friends Is another kettle of fish.
I would seriously suggest that you read the terms & conditions of PS via the link posted. As your arguing that It's ok to share your account details with a friend which Is clear In black & white not the case.