I recommend only doing this on test computers as enabling this in your enterprise could cause performance issues on your front ends especially early in the morning. Reason being is with this registry setting, the moment a Lync 2013 client logs on, it will pull down the address book (Right now in my environment it’s 40 MB). 40MB x 10000 users all hitting the front ends may be a bad idea.
One area where i could see being beneficial is within a Citrix/Terminal Services/Conference Room Computers like environment. Here me out:
Lets say your client policy is web and download. This would mean that a user launching Lync within Citrix would need to be only for 60 minutes to guarantee they receive the Address Book. In my environment many of our users use Citrix as a quick place to get info then they log out thus they may find the feel of Lync being different while in Citrix than on their desktop computers.
Again, you need to understand your end user population and how they use technology.