It's not a new SP, just a collection of post SP3 patches, hotfixes and the registry hack to allow it to receive some further updates. The updates are not being tested on XP by MS and many of the exploits being patched in newer desktop OS versions aren't being backported, so this could just give someone a false sense of security.
It may be a convenience for some users, but it's nothing really new. People have been writing scripts to do essentially the same thing for years.
I usually don't have a problem with people creating rollups. However, this particular case, like with most user rollups, they also include a lot of personal preference stuff like upgrading the browsers, messengers, .net fixes etc.
If I had a reason to maintain an XP "post SP3" image, I would use the default SP3 install (which does not include .net and only ie6) and only update that content along with hotfixes. I would leave installation of applications or IE upgrades to be done in Windows. I see that as a more clean way since Microsoft doesn't support native slipstreaming of IE or .net updates with XP.