Update: Note that while Office appears to install separate features, not separate products - it actually installs many separate products that do not display in ARP. A separate ARP entry is added to launch the bootstrapper installer that then maintains adding, removing and updating the existing products that you can't see in ARP.
These should be features, not separate products. Take the Microsoft Office suite for example, if you install the suite then you have only one entry in Add/Remove Programs and Word, Excel, Powerpoint, etc are all just 'features' off Microsoft Office and you update them that way.
However you can purchase some of these applications separately with their own installers and in these cases the "Application Core" as you put it is included with each installer, using shared components so that files don't need to be installed twice and each application can be installed/uninstalled independently of each other.
Here's a little excerpt from the Windows Installer documentation on Components and Features (you may also like to read Organizing Applications into Components)
Two components that share the same
component ID are treated as multiple
instances of the same component
regardless of their actual content.
Only a single instance of any
component is installed on a user's
computer. Several features or
applications may therefore share some
components.