If the files in question are pretty big I wouldn't necessarily rely on SMB to do the sharing, it's much slower than it's non-windows counterparts(namely NFS)
If there is going to be a lot of sharing of big files I would recommend you get a Linux box to do the job(and despite what any MSCE tries to tell you, Linux is INFINITELY easier to set up than any MS server garbage)
So speaks someone who does not understand what a server really does.
If all they are doing is simple file sharing then yes, it is overkill, and overpriced.
Windows servers can handle the sharing and there are several tools out there that will allow AFP on a Windows 2003/2008 server, we use one. Extreme Z-IP.
All the linux server people out there think servers are for file sharing, if you want to share files, get a NAS.
Servers are for managing your environment, running additional services and collaboration tools, etc, not just file sharing. Security, policies, automated setup, etc.
All things that Linux servers cannot even come close to offering what a MS server can. Their server products are rock solid as is their support of them.
I understand X server has decent management tools for the Mac but their hardware costs for smaller businesses are astronomical. The only financial advantage of Xserve is that it is unlimited users, so more users the more feasible it becomes.
Forget the emotional response to why Xserve isn't supported and look at the financial/business reasons. You need to hire someone with a totally different skill set, Apple's cost for on site 4 hour response warranty is absolutely insane, 20K per year. If you want phone support its $6K per YEAR!
HP's 4 hour response warranty on a decent sized rack server is around $700 and thats for three years, not one. Apple does not break out their hardware support, you must buy it as a bundle.
It does not make business sense.