I think it's important to listen to your spouse's opinion, when it comes to defending him/her.
In general, I do think it's a very natural impulse to feel protective when someone has insulted (or otherwise hurt) someone you care about. On the other hand, there are situations where a person's spouse jumping in to defend him/her from an insult won't make the situation better, and might make it worse.
Personally, if my husband wants to defend me from the things his family and friends say (if they say something that is insulting or hurtful), more power to him. For things my family and friends say, however, I only appreciate his jumping in to defend me if he's very subtle about it. As in, they don't really know what he's done subtle. Otherwise, I'd prefer to handle it myself.
I do appreciate my husband telling me (in private) when he was offended on my behalf, though. At least to a point. As long as he doesn't dwell on it, it's nice to know when we have both reacted negatively to something a person has said. It can quickly become counter-productive, but having him tell me offhandedly that he didn't appreciate what my sibling/friend/parent said to me can be very helpful. It has helped me determine more accurately when I am being oversensitive, and when someone has said something that is legitimately off.