Do supertweeters matter when we cant hear past 15,000 hz?

Sorry to be late to the party Veroman, but better than never?

While I think that just about everything already said is true, there is another aspect to consider: the crossover.

As Ralph (atmasphere) points out, rolloff causes phase shift, which is very bad. More generally, all RLC or variant circuits cause phase shift. All analogue filters consist of RLC circuits. All crossovers, whether in the speaker or in an analogue "electronic" crossover, use filters, and so cause phase shift. Hence all analogue crossovers cause phase shift.

If you are limiting the super tweeter (to protect it from higher energy low frequencies) with an analogue high pass filter, it too will cause a phase shift. That is one important reason why full range ESL's sound different from any other speakers - they have no crossovers.

Hence super tweeters might sound better to some people in some systems, but the benefit comes at the cost of phase shift. I suspect that the most bang for the buck is in quality caps and resistors and inductors in the crossovers; that is, if you don't spring for ESL's. YMMD

Tweeter will produce phase shift on its own without any analog filters.  It is related to weight and stiffness of moving element.  

Therefore....is the addition of a super tweeter a benefit to my speakers

Yes, it is.

I have more practical question: Coral h-104 vs Fostex T900a vs T500amk2 - which one are better as a pure tweeter for Avatgarde Due horns. Or may be something else is better?