Using the ShellThumbnailing, also called thumbnail generation through WindowsApi, creates a dependency from MediaPortal.Common to
 - Microsoft.WindowsAPICodePack.dll and
 - Microsoft.WindowsAPICodePack.Shell.dll

The result is that these dll files required running every MediaPortal application (i.e. ServiceMonitor) which needs the Common library (i.e. for logging).
It is also not possible to get rid easily of the shell thumbnailing if a user does not want to use it.

There might be additional reasons, i.e. easier replacement if there are better thumbnail generators available than using the windows API.

In the first change it might not be need to implement new interface to make the thumbnailing more flexible. It should be enough to start moving the thumbnailing code into it's own plugin and create dependencies from other plugins which are using this code.