Various versions of MS.Net are required for certain programs to function properly...my experience is that, if required, the program download/install instructions will notify the user that said version of MS.Net is required.http://www.w3schools.com/ngws/ngws_intro.asp
For example, I know that ATI drivers (download) and nLite each require some version of MS.NET to properly function...because the respective sites inform me of such.
My approach is to only install any version of it...when I am sure that it needs to be installed for some function. Otherwise, I don't install it.
MS.NET is not a critical download, it/updates for it...appear at the WinUpdate site as an optional software download (on my systems).
Critical downloads are seurity related, whereas MS.NET is not (except for possible security-related updates to it, once it is installed).