[dmix]

Here is one interesting discussion:

https://twitter.com/thegrugq/status/399352954060144640

The author has cited terrorists training manuals elsewhere on his blog, that are apparently available publicly online, dated from as early as 2003 with security guidelines to not use email or talk on cellphones.

TLDR: The adversary can easily stop using email/cellphones to discuss plans. Do they still use email/cellphones for other reasons? Sure most likely, as was shown in Zero Dark Thirty, but not in any meaningful way that can be usefully gleaned from a mass-surveillance approach. Therefore the large investment and privacy trade-offs to the greater society isn't worth it.