But in principal, I can see why a company would think any and all info they have might be useful in a dispute.
Let's say that the carriers only stored call records with tower IDs for each call. If there was a dispute every so often because a call was made on a close-by tower that itself happened to be located in a different toll bracket (resulting in an incorrect overcharge to the customer), I'm sure the carrier would happily write off that charge if you disputed them about it, which is probably what they'd do right now anyway.
There must be another reason they're keeping it, even if it's just a case of it being super-cheap to store, and they think that they might figure out something to do with it later.
http://news.ycombinator.com/item?id=2371548
The discussion included that they use it for modeling traffic patterns and areas to help them plan capacity and new towers. I'm sure they use it for other things as well.