Maybe, but it sounds more messy. Verizon would have to maintain a semi-public (to their "other" customers, that is, advertisers etc.), low-latency queryable dat

Maybe, but it sounds more messy. Verizon would have to maintain a semi-public (to their "other" customers, that is, advertisers etc.), low-latency queryable database of {source, destination, customerID} tuples, and ads and/or pages might not finish loading until that query returns. If the ID is more static, the trackers can cache the identity information for awhile after the first time, and Verizon probably has less work to track and resolve IDs.