Here's a nice summary of the benefits of developing Office add-ins with the new VSTO AddinLoader. Security and administration of add-ins was clumsy in the old days (i.e., Office 2003), but things have changed. Good riddance, mscoree.dll.
That post is on David Gainer's Excel blog, but it was written by Patrick Smith, a PM on the Office Programmability team. I like the way some of the most popular Office-related bloggers have started running guest posts from others on the office team, such as Savraj Dhanjal's excellent UI programmability series on Jensen Harris's blog. Nice work, guys!