Ribbon midrange pros and cons


Ribbon tweeters are fairly common on some high end speakers. ...they sound great.....can anyone tell me why ribbon midranges are hardly ever used by some of the best speaker companies. ..what are the pros and cons? 
128x128vinnydabully
Give some examples of a speaker with such a beastie. :) 

Red Rose Revelation
Infinity had the EMIT tweeter and EMIM mid back in the day; both ribbon diaphragm design. Innovative at the time but not a big hit.
bsmg
Infinity had the EMIT tweeter and EMIM mid back in the day; both ribbon diaphragm design. Innovative at the time but not a big hit.
Some of those speaker systems have withstood the test of time, such as the RS-1 and IRS Beta systems. But they aren't ribbons - they are planar drivers.
Wikipedia describes the Infinity emit and emim units as "quasi ribbon" units so it would seem I was quasi correct.