I think that you may be right,,.and wrong .
Microsoft Update...will handle all the MS apps and I go to Microsoft Update...but I never download anything but critical updates. Critical updates are not restricted in applicability to just XP, MS has long issued critical updates that apply to multiple versions of Windows.
MS.NET is a Windows component, it is not a separate application such as Silverlight, Office, etc. It's part of the O/S and a critical update for MS.NET is categorically a critical update for the O/S. Same for IE.
The point is...critical updates, regardless of what versions they apply to...are occasionally released for XP users. Something other than the Malicious Software Tool has been released as a critical update for XP users, during the last 2 years...several times. I don't know if this will continue but it has happened.