Microsoft think it's "important" that bods have the latest versions. That is their mentality. But unless you are running software that is dependent on later versions of .NET (or if MS stop issuing security dates for older versions) you do not need to install them. In straightforward terms .NET provides software functions for applications to use (rather than developers writing their own, and so makes development a lot quicker and standardised). If you install something that needs a later version of .NET , the software install should tell you.
I have .NET 3.5.1 (as that is what Win7 SP1 came with). In respect of .NET I only install updates that cover 3.5.1. I hide all other .NET updates (by right click on the item in "Select Updates To Install"). Never had anything demand a later version.
And I agree, that Important doesn't seem to be the right label - Recommended would be more appropriate.