I would like to have an extra field too... I mean: having the possibility to add it. Anyway, I bet that most of the people who have asked for it are thinking about the same as me: birthdates. I am a teacher as well and I want to know the birthday of those of my pupils who are using the forums, and not only that: I want to know their age. I don't need it to be a required field, but what if I needed the default fields as well?
If it is such a pain to code that feature I guess we can "live" on renaming any of the messaging fields, but I do think birthdates and, as I have read, "talk about you" are useful in forums, to socialise, so in my honest opinion it would be interesting to have such a field by default.
As for the rest, I love PunBB 