T-mobile was the last of the big four networks to get 3G up and running and I expect that they will be the last to get LTE-Advanced up and running. As a customer, I do not have a problem with that. Its the price I pay for lower monthly fees.
T-Mobile will likely never get LTE-Advanced running since they don't have the spectrum to deploy it. They didn't have the spectrum to deploy HSPA until the AWS bands were put up for auction; by that point in time, the other major carriers had their 3G networks deployed or in the process or being deployed.
AT&T does not need full control over all UMTS frequencies allocated in this country (which is what a T-mobile acquisition will give them). If you think that AT&T is going to start giving out 1700 MHz phones to people then you're mistaken. The 1700MHz bands will likely see primary use as a premium network where they can charge extra for the available bandwidth. A lot of OEMs of box tops are interested in going this route for their devices.
AT&T has already stated that if the merger goes through, the AWS bands will be used for LTE.
That said, it's rather obvious AT&T doesn't need T-Mobile as much as they say they do. If anything, T-Mobile needs AT&T. The severance T-Mobile gets from AT&T if the merger doesn't succeed is pretty much the only infusion of cash they'll see while their profits dwindle. If and when (more when than if) T-Mobile collapses, the end result would be more or less similar to if the merger did succeed. IMO, the merger may as well succeed.