Do tweeters with open rear chamber need enclosure?

What is the speaker in question?

"Not when used in a dipolar topology as Alon/Nola has been doing for years with VERY good results..."

But they still need a baffle or sound waves will cancel.

Even modestly crappy speakers have the woffers, highs, mids in a seperated inclosure.

"But they still need a baffle or sound waves will cancel."

Are you familiar at all with Alon/Nola speakers? Those models that employ the dipolar topology for the midrange and tweeter drivers have the drivers mounted on a baffle.
The Alon line had a seperate baffle of the mid/tweeter driver array mounted on top of the woofer enclosure but the Nola line uses a common baffle.

Do a GIS for Alon/Nola and you'll see what I mean or you can go straight to their website: http://www.nolaspeakers.com/