I'd hedge my bets and say there is more to it than that, and this is one of the reasons why uninstalling DirectX has never been officially supported and is rather hit and miss with third party applications.
Well, why would you want to uninstall DirectX anyways?
No DirectX, no gaming.
Myself its a been there done that kind of thing. One might say over and over Live and learn.
Don't worry, you're not alone. Can't recall how often I've murdered my Windows installations.
Fortunately, I'm also a fairly good medic. :d
Alright, let me have a hit at it.
If you update your DirectX, do you see a seperate entry for DirectX 10 on Microsoft's homepage?
No.
Why? Well, if you obtain your DirectX via a redistibutable file (all files in one archive; no internet connection required for the installation) and not via Windows Update or the DirectX web installer, you will notice that this archive includes DX9c *and* DX10 files.
I'm pretty sure the installer determines your windows version (XP, Vista) and only installs those DirectX files appropriate, or rather MS-approved for your OS - DX9c for XP, DX9c and DX10 for Vista.
Now, all those developers have done so far is taking the DX10 files from the common DirectX redistributable package and putting it in their XP installation. I don't know how they got their hands on the registry entries for the new DirectX, but I guess they have either taken them from the DirectX installer or "reverse-engineered" them from a Vista installation.
Of course, there's always sources for errors. A missing .dll, a bugged registry entry and, mostly I guess, video drivers for XP not suitable for DX10 operation, because the developer left out DX10 support for a non-Vista driver.
All in all, DirectX is just a harmless dictionary making 3D applications and the video driver able to communicate with each other. Nothing more.
This is partially proven by the fact that you don't have to reboot your PC when you install DirectX. If you don't have to reboot, it can't be essential for Windows operation, now can it?
I don't want to blackmail Microsoft, but as far as I'm concerned, DirectX 10 was nothing but a marketing argument for switching to Vista.