Modal music has rules that go beyond simple the names of the scale, and if the music does not follow those rules(or attempt to follow them, as, since Gargoyle's post above points out, the modal system was, artificial) it isn't modal--that is my point, pure and simple--

It is fine to say that a song uses a Dorian scale(which, by the way, Greensleeves, doesn't--there wouldn't be a leading tone in a Dorian scale, C# if you are playing it in the key of Dm, which there most assuredly is), or that Old Joe Clark uses a Mixolydian scale, though someone could argue that it is simply a dominant scale--

I don't make this point about what the proper terminology should be to be a jerk, I make it because it is necessary to avoid confusion--there are folks who frequent this place who play Lap dulcimers and all the variety of pipes, and there are folks who come around that play early music, and those who sing and perform old church music, and there are a few who play Greek and Middle Eastern music, all of which use modal music of one kind or another(Banjo can be modal, too)--

BruceO(may his name ever be praised), used to remind us, periodically, that there were one hundred and sixty or eighty odd scales that had been found to be used in British Isles/or what every you want to call it, folk music--the "Greek Mode" names were grabbed as a convenient way of describing some of these--but they are rough approximations only--and there is a lot swept under the rug--