My best friend in high school used this once. The condom broke and she wasn't going to take her chances. This was before it was over the counter and she didn't feel she was in a position to get a prescription (very religious strict family). She found someone that knew someone and while she was miserable and sick for a few days, she told me that she felt better knowing that she was able to eliminate the possibility. This was actually pretty responsible on her part, as at the time she was with her (then) boyfriend, in a very monogamous (long term) relationship.
I've got to agree with what everyone else has said here. This shouldn't be an excuse or a freebie. (Like I've heard adolescents say they don't care about getting AIDS because there's medicines for that and they'll live their whole lives...
) It should be in case of serious accidents or events.