This aggregate data is the same data that Facebook sells to advertisers to bring in their revenue. There isn't really any damage in AT&T selling this aggregate data, though concerned users can opt out. If you are worried that AT&T's data may not be completely anonymous or are concerned about your privacy, we recommend you take AT&T up on their offer to opt out.
You might be curious as to why AT&T didn't attempt this before. Part of the reason is that the older networking gear was proprietary and the data wasn't easily accessible. This same gear has caught the big data bug and its data is now much more accessible.