I don't think the Tormach PC was ever intended to be a general purpose PC. It's purpose is to be a machine control PC, and nothing else.
The definition of 'general Purpose' seems to be a stumbling block. Lilkely if you ask 6 people you could get 6 answers.
So long as Tormach is still on the table, I can say for certain (because I have correspondence from Tormach management that says as much) that Tormach does not want their customers using anything non-Tormach and that they do not condone or support their users modifying the systems. In fact they go out of their way to prevent it. So for those users who are willing to be confined to that playpen, it may be accurate in a sense to refer to their 'CNC controller' as 'special purpose', but that would be because Tormach has made it so, not because the PC itself has that restriction. My opinion only, of course.
I would venture to say that the vast majority of MACH users run more programs on their CNC PC than bare bones MACH3. In my experience, MACH users have a multitude of add-in programs, plug-ins, estimators, calculators, editors, emulators (i.e.ost USB devices) . Further, while a user might not 'Surf the NET' using their 'CNC controller', there are a growing number of TCP interfaced products (smoothstepper, for example) that require the same network services as the browser.
The only reason (other than cost saving) to use an Embedded OS is to trim it down by leaving out modules and services that are not needed for a particular application. Building a special purpose OS is made simple by Microsoft by slicing up the Windows OS and putting in in an ala cart cafeteria line. You push your tray down the line taking the stuff you want, but only you know what's on the tray at the end. If someone comes along to use your OS and they need a peach cobbler routine but you only took the apple pie off the dessert shelf, then they are not going to be able to run on your system.
I have two 'CNC controllers'. One for my InTurn™ 4th axis and one for my ATC. Neither has a conventional OS, embedded or otherwise. You cannot run a windows program on either of them, for example. They have their own code and they carry out their own tasks, period. They are indeed 'special purpose', but nobody is going to want to run a speed and feed calculator on them, so the inability to run a windows program is moot.
On the other hand, Embedded Windows is going to lack something that the full OS has (otherwise, except for cost, there would be no reason to use it). The reason I think it is safe to say it is the wrong choice for a CNC PC for running MACH3 is that unless you have some documentation describing what has been left out, it is a crap shoot every time you want to add something to MACH or utilize an ancillary program that would be beneficial to run on the CNC host machine, even if it is only a driver for an ad-in PCI parallel port, which incidentally is an actual example of a program that will not run on the Embedded OS. The program is calling services that do not exist and if the programmer included a trap for that, the program would exit gracefully with an error message. But why would a programmer do that? It is as likely that the application (or driver) would crash the OS thru a deadly embrace, crash a running app (MACH3 for example) or simply destabilize the system. rams. With an Embedded OS, there would be no way for a user to diagnose this situation (that I know of). Again, just my opinion.
As I said earlier, at one time there seemed to be some valid reasons to create a PC with restrictions in order to remove some of the Windows 'background noise' that caused timing problems or other issue with MACH. As I learn more about this topic, I become more convinced that the published reasons (now 5 years old) for using an Embedded OS are no longer valid. Tormach is the example, only because they are the only major supplier that I know of that provided Embedded Windows (and a very old version at that) and also published their reasons for doing so.
The problem with using Tormach as a barometer is their all encompassing KISS principal. That might be the only driving force behind their continued use of Embedded Windows, so the fact that they still use Embedded Windows does not necessarily mean they have any technical basis or advantage for doing so. It could simply be their extreme aversion to change. That's why I was seeking other opinions and advice that might be more current and not be laced with any particular vendor's ulterior motive.
The above detail may clarify my position, or it may simply further agitate the locals, however, on reflection this forum was probably a bad choice on my part as a place to get such information. The mention of Tormach here, in almost any context, brings angry peasants bearing torches at the castle gate. Once that happens, there is no getting in or out until you give them the monster they seek.