If a terrorist cell is planning an attack, and they suspect that their activity is being monitored, then they might switch to a different cell and/or way of communicating... and you lose your source of info at a critical time.
The reason why comm intercepts have always held the highest classifications, is because knowing any piece or capability of it, can be an aid to whomever is being intercepted.
World War II had an enormous number of examples of how intercepts (and code breaking) without the other side suspecting, changed the course of history. E.g. Japanese Naval code and Midway. German Enigma and all that entailed.
The downside is that such security can be abused. Nixon took advantage of COMINT security protocols by originally classifying his tape recordings with the highest level codeword, something that was more intended for securing foreign derived information.