The Chasam Sofer writes that the reason Jews do not speak Loshon Hakodesh as a speaking language is because it is inappropriate to use a holy language while enveloped in Tumah, which is our current status. The Rambam writes that a love song in Hebrew is more repulsive to Hashem than the same song in Arabic, for instance, because the pollution of the Holy language is an additional crime. If someone wants to store pornography in his house, thats bad enough. But to store it in the Aron HaKodesh is unspeakably worse. So to cause Loshon HaKodesh to be used as a street language, complete with all the disgusting ways it is used today in Israel, is just more of a reason why we should make sure it never gets into the streets. For our Creator to look down at the world and see His holy language – or even elements of it – used in magazines such as are sold in Kiosks on Yaffo or Dizengoff Street, or spoken by the lowest of the low trying to make a sale, is not something that he or we are happy about.
The Kuzari writes that Avrohom Avinu, therefore, spoke 2 different languages. One for holy speech – that was Loshon HaKodesh, and the other for mundane speech – that, the Kuzari says was some non-Jewish language that Avrohom Avinu took and changed around a little on his own. And thats the idea behind Yiddish. It is a non-Jewish language that we took and twisted a bit in order to make it exclusive among us.
Even though there are Yiddishistin who speak Yiddish, they took it from us, not vice-versa (as is the case of Modern Hebrew), and since we do not live in a country or society dominated by Yiddish-speaking shkotzim, there is no benefit of Lo shinu es leshonam by not speaking Yiddish. But there is such a benefit by not speaking Hebrew.