apt pinning confusion

["Todd A. Jacobs" <nospam@codegnome.org>]

Historically, I've always used APT::Default-Release to keep my system
sane with multiple repositories, but recently reinstalled a system
because it was getting very crufty. I'm trying to prevent a similar
recurrence, so I now have:

    $ cat /etc/apt/apt.conf
    APT::Default-Release "testing";
    APT::Cache-Limit "25165824";

    $ cat /etc/apt/preferences
    Package: *
    Pin: release o=Debian, a=testing
    Pin-Priority: 800

    Package: *
    Pin: release o=Debian, a=unstable
    Pin-Priority: 700

    Package: *
    Pin: release o=Debian, a=stable
    Pin-Priority: 600

    Package: *
    Pin: release o=Debian, a=experimental
    Pin-Priority: 550

I also have some additional sources (e.g. security.debian.org) which I'm
assuming are handled properly by the apt defaults. My question is, have
I set things up properly to do what I'm expecting? 

I'm particularly unclear on whether installing something out of unstable
starts tracking that package out of unstable, or whether it simply adds
the package until an equal or higher version is available in testing.

Lastly, is pinning even really necessary here? How does that help me
over simply setting the default release? In practice, I haven't really
seen a difference yet.

-- 
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	-- Doctor Who, "Destiny of the Daleks"