see this - https://jamfnation.jamfsoftware.com/discussion.html?id=5416 - It talks about 10.6, but I've used it on 10.7 and 10.8... and since I'm seeing it in 10.9's, I'm starting to move the file into there -
In certain, situations you never want use a clients local disk for the NetBoot shadow file, like if you want to run hard disk utilities like Disk Utility or Disk Warrior, or for us if we want to run the file system maintenance software we use, radmind, out-of-context. Where we boot from NetBoot, but we update the file system on the clients local drives.
Also, due to the limitations of number concurrent Apple File Protocol [AFP] connections using Mac OS X 10.6.x (and others) client's File Sharing. If you need to NetBoot more than 10 clients and run maintenance utilities, you can't use a network shadow file, because after the tenth client connecting, it will either try to use a local disk for the shadow file or not NetBoot properly.