Why does kmail2 change location of mail storage without
informing the user?

I now find my most recent sent mail is located at

~/.local/share/akonadi/file_db_data

and NOT at

~/.kde4/share/apps/kmail/mail/sent-mail

I have ~/.kde4/share/apps/kmail/mail set as local folders
mail resource

This makes backup much more complex. In fact, except for the
fact that kmail2 is so riddled with errors, forcing the user to
dig into what is going wrong, I would not have had any reason
to think my mail was being stored in the wrong location.

My question is just: why is this relocation happening?

It's tied up in the long term objectives of the KDE project. See for example

http://community.kde.org/KDE_PIM/Akonadi

It's tied up in the long term objectives of the KDE project.

OK - I don't mind being a beta tester, but I've switched to
thunderbird as my main emailer

Suddenly, filters work consistently, I know exactly where all
my mail is stored, I can keep local mail perfectly synced by
simply rsyncing the mail folder, my email program starts and
stops quickly, I can efficiently use my email program across
an ssh connection, etc. etc.

kmail looks prettier though