[Samba] group changes?

[Adam Williams]

I have several groups in /etc/groups on my samba server.  I upgraded to 
samba 3.0.23b and then users couldn't connect to any shares they were 
groups on in /etc/groups (shares used valid users = @groupname) but 
could connect to shares where valid users = their_username.  I did some 
reading and saw the group mapping change at 
http://us5.samba.org/samba/docs/man/Samba-HOWTO-Collection/ChangeNotes.html#id2550114

And then I did net groupmap add unixgroup=mygroups and that worked

Can I ask, why the change, and why do I have to execute that command for 
each group instead of samba just getting who's in the group from /etc/group?