Software developer hat on.
The issue with everybody choosing a different extension, or of having a
plethora of extensions for different types of data that can be stored in
compliant OpenEXR, is that it's a pain in the behind for apps that want
to use the file extension as a clue for what format it is.
For example, consider an application that has a "file open" dialog that
when choosing a texture file will highlight image files, and it knows
about ".tif", ".exr", ".jpg", and so on. If somebody decides that in
their studio they want deep exr files to have the extension ".dxr", and
exr files that have some some deep parts and some non-deep parts to be
".cxr" (complex), and stereo frames to be ".sxr", and MIPmapped exr's to
have ".mxr" to distinguish them from flat exr's -- this could go on
forever -- well, that file dialog is going to annoy the crap out of people.
So I give +1 for having standardized file extensions correspond
one-to-one with file formats, NOT multiple extensions for different
corner cases of the same format.