In the case of the NYC market, AT&T had the worst spectrum planning of any major carrier. They are suffering because they chose to rely on low-band spectrum rather than densifying their network, and are now trying to band-aid the solution by using carrier aggregation of oddball downlink bands. It doesn't change the core problem of having too many people on each sector.
The reason they have to stake 2G dead this early is because they are spectrum starved and already only running it in HSPA guard bands. They need every resource they can get since they haven't been spending money at throwing towers in every possible place in the city for years. They've been using it to buy a satellite company, a Mexican carrier, and WCS spectrum that's worthless without densification. Priorities.