The default action on *.jar files should be to let JAVA run them because, as you say, they are JAVA.
The vast majority of people shouldn't be looking into *.jar files since they are never used as a means to package files form delivery but as an executable. I'm surprised you even phrased your comment the way you did, with mild sarcasm.
By all means allow MC the means to open them as an archive, BUT NOT as default.
The open-source document formats like *.ods & *.odt from programs like LibreOffice or OpenOffice are also compressed archives, using your logic, the default action for these will also be to open them in MC as an archived folder and examine the contents! I'm sure you agree that is the wrong thing to do.
I was not aware that you have received lots of requests to implement this function but, I'm sorry, regardless of the number of requests this is not the best way to treat those files.
The default action on ANY file, ie. the action performed by a double left-click, should be the same as the action Windows would perform on it if invoked from the commandline.
Call a *.txt file and the default action is to run the assigned text editor, call a *.jpg and your graphics editor/viewer will run, a *.zip and your archive program handles the file. Calling a *.jar would call the associated program. If you don't have JAVA then it probably would be an archive manager, but 99% of the time it will be JAVA.
Just how hard is it for people that want to do any non-default action on ANY filetype to setup a context menu action.
I hope you read this in the context in which it was written, not as an angry response, but as a bemused MC user that would like you to reconsider this position on the default actions for specific filetypes.