Have you played with Windows Home Server Beta ?
Not the new one but I've worked a little with the old one. It is still not a true business solution.
Windows servers are not about file sharing, if all you wanted was a simple file share, buy a NAS or a cheapo Linux box with a bunch of space, or as you mention WHS.
AD is about management of an IT infrastructure. A properly setup AD environment(and not many are setup correctly) will reduce costs massively in managing a larger infrastructure.
60 machines is a huge number to manage. When you start to look at compliance and legal requirements that are becoming the norm today you need a management tool, and a good one.
Massachusetts passed a law that became effective on March 10th. Business owners that manage people's personal information not only have to encrypt the data but they also are required to keep their infrastructure up to date.
Security updates, security policies, antivirus updates, all of those things the business owner now must keep record of in case of a breach.
A solid AD setup makes those kinds of things a breeze. The server manages the updates, makes sure all the machines are compliant, makes sure users don't do what they are not supposed to do, etc.
Look at something as simple as WSUS(Windows Server Update Services) on a domain controller. He has 60 machines and a 35mb patch is released. Imagine 60 machines in a business all trying to download the same patch at the same time.
Now what if that business was on a VOIP system, traffic havoc.
WSUS gets the update once, disperses it to all the machines, overnight with WOL if you want, then reports back if any failed. No users telling it not to do the update, no checking 60 machines to make sure they were all done, etc.
There are 20 situations like that in a company with 60 computers.