I got the following entry from my Technet Events Bloggers feed.  It seems that local copying of the Offline Address Book for Outlook can have serious consequences for network performance.

Heck, I've wanted to throttle the OAB lots of times - but that's not the reason for this blog entry :-).  Chris had a challenge as the customer he was doing some work for wanted to manage the network bandwidth when all of the users in the company were downloading a full copy of the Offline Address Book.  When this happened, network activity went a bit crazy and the WAN became unworkable (not surprising - it was a big company).  Chris was trying to work out a way so that not all of the clients could download the OAB at the same time.

I guess that this is where having field level replication in Notes is a supreme benefit.  In all my years as a Notes professional, I can honestly say I have never thought about the impact of replicating the PAB to a user's local machine.  I guess this is either due to the fact that either Notes does this right out of the box or I am one heck of an administrator.  I am voting for the first choice.  The article has a few links to some help documentation on how the OAB works, which might be very interesting to any of those Domino folks living in an Exchange nightmare.