Windows Server 2008 introduced read-only domain controllers, which receive a full replica of the domain database but can't modify it, just like a good old Windows NT BDC.
I know all the technical ins and outs of how to run those semi-DCs (I just passed 70-646 and 70-647), but still I don't have a clear answer to the most important question of all: why should you use them?
This comment from TheCleaner really sums it up for me:
@Massimo - yes, you are correct. U are looking for a compelling reason for an RODC and there isn't one. It has a few additional security features to help alleviate branch office security and really only needs to be deployed there if you don't have a DC there already and are anal about its security.
That was the same I was thinking... a little increase in security, yes, sure, but definitely not so much to be worth the hassle.