It's not clear that everyone can learn not to be offended, and being offended imposes costs on the group in terms of the things they're going to consider and share.

But assuming that everyone could learn not to be offended:

Everyone can learn not to be offended by the few offensive people, or a few offensive people can learn to be less offensive. The group where the latter holds has made much more efficient use of its time and can work with a wider range of other groups.

So, for all you need people who can discount a certain level of offence in order that they can share differing ideas, I don't know whether don't be offended is the most efficient group norm to put in place for dealing with cases of disrespect and/or wilful offence.