A lot of speakers have ribbon tweeters these days, it not really uncommon at all. Like any other technology, some people like them and some don't.
why aren't ribbon tweeters more popular?
What is the downside to ribbon tweeters? I haven't had any in a long time (Phase Linear P580's, stolen, still miss them.) Ribbons offer great extension and speed; other than expense, what objectionable qualities limit their use?