When you use Outlook automation, you are scripting ... well, Outlook. Which says that your script will use a running instance of Outlook or maybe even start one.
When you directly attach to MAPI, you are acting like any other client using this interface (like Outlook does too).
That said, there are certain things you might only be able to do with OL directly, while other things can better be done with MAPI directly.
When I originally worked out for me the MAPI access presented in the second node you linked to, I was looking for a way to access an Exchange server mailbox from a Windows service, which deemed unlikely to be done by scripting OL to me.
When you miss a feature in OL automation which is doable with MAPI access, what would conversely be the feature you can do in OL scripting you miss the other way round?
On another note: Please reread "What shortcuts can I use for linking to other information?", with respect to linking inside perlmonks.
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