In addition to Otaku's links, this shows an example (near the bottom) of navigating an OpenXML document using the IO.Packaging namespace versus the SDK.
Just like Microsoft states on the download page for the SDK:
The Open XML SDK 2.0 for Microsoft
Office is built on top of the
System.IO.Packaging API and provides
strongly typed part classes to
manipulate Open XML documents. The SDK
also uses the .NET Framework
Language-Integrated Query (LINQ)
technology to provide strongly typed
object access to the XML content
inside the parts of Open XML
documents.
The Open XML SDK 2.0 simplifies the
task of manipulating Open XML packages
and the underlying Open XML schema
elements within a package. The Open
XML Application Programming Interface
(API) encapsulates many common tasks
that developers perform on Open XML
packages, so you can perform complex
operations with just a few lines of
code.
I've worked pretty much only with the SDK, but for example, it's nice to be able to grab a table out of a Word document by just using:
Table table = wordprocessingDocument.MainDocumentPart.Document.Body.Elements<Table>().First();
(I mean, assuming it's the first table)
I'd say the SDK does exactly what it seeks to do by providing a sort of intuitive object-based way to work with documents.
As far as automatically patching the relationships -- no, it doesn't do that. And looking back at how you actually state the question, I guess I might even say that (and I'm fairly new to Open XML so this isn't gospel by means) the SDK2.0 doesn't necessarily offer any extra functionality, so much as it offers a more convenient way to achieve the same functionality. For example, you still need to know about those relationships when you delete an element, but it's a lot easier to deal with them.
Also, there's been some efforts on top of the SDK to add even more abstraction -- see, for example, ExtremeML (Excel library only. I've never used it but I think it does get into things like patching relationships).
So I'm sorry if I've rambled a bit too much here. But I guess my short answer is: there's probably not extra functionality, but there's a nice level of abstraction that makes achieving certain functionality a lot easier to handle -- and if you've been doing it by hand up until now, you'll certainly have the understanding of the OPC to understand what exactly is being abstracted.