Whenever a user joins a community, his/her "about me" section is filled with that of his/her oldest account. Though it may be a proper way to make a verbatim copy of "about me" section to the new community instead of leaving it blank, I think it is not desired always (or maybe mostly). Let me explain how:
We've many categories of sites viz.: Technology, Culture/Recreation, Life/Arts, Science, Professional, Business etc. Now, "about me" on different site of a different category would be obviously different! In addition, some users also write different "about me" on different site of the same category.
So, shouldn't we just keep blank "about me" section of new joining community instead of copying it from doest account without prompting? Of course, we can redirect/prompt user to the edit area of about me (site.stackexchange.com/users/edit/[user_id]) so-that user get remind to fill it.