"The Coolness of AEC", Justin O'Conner from Biamp:

AEC’s benefits can be difficult to quantify. “Improved communication: $3” does not appear on a financial statement.

A diagram of how AEC works

Yes it is expensive and when it's all setup and running the client will ask where the additional expense went as everything sounds normal. That's the point. There is an incredible amount of processing and science that is needed to effectively cancel out an echo from distance communication when using a medium to large scale audio reinforcement system to maintain a true bi-directional conversation. Heck it's even hard with small portable units too.

If there are two people in a room or three, if a door opens or closes, if another person joins, if an object or a microphone is moved, if a person speaking leans closer to a microphone

Do you ever think about these things when you are participating in a Skype call or Google Hangout?