One of the ways that desktop as a service providers get around Microsoft's crazy Windows client licensing rules is to run Windows Server OSes tweaked to look like a client OS instead.
Microsoft VDI licensing rules for client OSes require each customer use Windows 7 or 8 on hardware dedicated to that customer, not to mention expensive Virtual Desktop Access (VDA) licenses and many other complexities.
Using Server instances is a lot cheaper, but don't let it fool you. The desktop as a service (DaaS) providers who do this aren't offering terminal services and calling it VDI; they're delivering a dedicated instance of Windows Server to each user as if it were Windows 7 or 8.
How Windows Server saves on Microsoft VDI licensing
Windows Server has a Service Provider License Agreement (SPLA) which requires the end user to buy only Remote Desktop Services (RDS) Client Access Licenses (CALs). That's much more cost-effective than VDA licenses. Those RDS CALs are typically so cost-effective that they're rolled into the monthly cost per user, even for the dollar-a-day providers.
If a customer wants to roll out Windows 7 or 8 instead of Server however, both of these advantages are thrown out the window. The monthly cost for Windows 7 desktops might be the same as Server, but you have to bring your own licenses. The extra money that the provider "saves" because you're getting your own licenses is actually spent on more hardware.
Not everyone is as sweet on DaaS as I am, so let's look at it from the on-premises side. Though you don't have to care about things like multi-tenancy (multiple customers running on the same physical hardware) when you're going on-prem, licensing still burns a hole in your pocket. If you're doing VDI, you're probably already aware of how expensive VDA can be. But you can take a page from the DaaS providers' book and deploy single-user virtual machines (VMs) running Windows Server that users access via RDS CALs instead of VDA licenses.
The key to this situation is that from Microsoft's perspective, you're not doing VDI because you're not using a client OS. Rather, you're deploying single-user Remote Desktop Session Host servers. And because they're single-user, you don't have to worry about lockdowns and isolation. Your users can have admin rights and everything else they might expect from a client OS.
How to do it on-premises
To pull it off, you'll need to purchase the Datacenter edition of Windows Server. With the Datacenter edition, you can deploy as many instances of Windows Server as you want. The $6,000 price tag might seem steep, but once you spread that out over all your users it becomes quite reasonable. After that, you need only to purchase an RDS CAL for each user.
The end result, as detailed in Falko Gräfe's white paper on the cost benefits of single-user Windows Server VDI desktops, is that the Server instances cost nearly 60% less over six years than the client OS, VDA-licensed alternative. In the example in the white paper, each Server user will cost a total of $633 over six years, but using a client OS with VDA will cost over $1600 over that same six-year period. That's just one example, and everyone's cost will be different, but the concept is identical.
VDI with client OSes can be cheaper depending on whether you've purchased Software Assurance, but VDI with Windows Server VMs always comes out as the cheapest overall option. It's one of the reasons DaaS providers do it, and if you can't come to terms with the exceptionally high licensing cost for VDI in your own environment, you might consider doing it too. It won't take long to realize the cost savings.