The class .embed-responsive-item could be use if you want to apply the style to another element that iframe, embed, object.  Personally i don't have a use case and never use it for now, it was there in the code from @necolas i just leave it.

I believe we should keep the stack of iframe, embed, object, the main use case of this it's to embed video from youtuve, vimeo...  if you remove theses elements then you have to modify the code you grab from youtube, vimeo...
Also in some case  you cannot modify the attributes for 3rd party widget code.

Your call  i will update the PR with your feedback