There are a number of reasons, but back in the 70's AT&T was a monopoly on land line services, and eventually the government forced them to break up, and therefore you had many companies that were called Bell this or Bell that. In one example, this could be Bell Atlantic (now Verizon). By forcing competition, you have unlimited domestic calling plans and features that would have otherwise been too expensive.
The same applied in cellular service. Since AT&T can't buy out most of their landline rivals (which is pointless nowadays), they are going after the cellular network. In the US, there are 4 major companies that own their networks. AT&T, Sprint, T-Mobile, and Verizon. All of the other carriers are regional, or VMNOs. Out of the big four, only T-Mobile uses the same cellular technology (GSM) as AT&T. Therefore if they were to buy any carrier, that would make the most sense. However, to do this, it would mean only 1 GSM carrier which could cause problems as GSM is a global standard.
In addition, out of the big 4, T-Mobile has been the one customers tend to like the most. They were the first carrier to offer a RIM device, and had the first 2 Android phones on market. They also were the first to offer an unlimited calling plan that was less than $100. They also offer the idea of a family circle, and even walked into UMA services. One could say that when the other carriers offered this, it was because of T-Mobile. In reality, T-Mobile has been the company to shake up the status quote. Once T-Mobile is gone, it is not likely that a carrier will have as much of an impact. In addition, AT&T (as well as Verizon) are the two most expensive carriers in the US. They have the most customers only because of their coverage.
Now, if AT&T owns T-Mobile, they will eventually kill the 3G towers and spectrum to make room for the LTE capabilities. This means that every T-Mobile phone that uses 3G or HSPA+ (marketed as 4G) will be reduced to 2G. This means extremely slow data connections, and no simultaneous data/voice services. AT&T will most likely allow customers of T-Mobile to keep their plans, but they could just as easily tell anyone wishing to get a phone that will work on 3G to pick one of their plans which is about 30% higher than T-Mobile's. And there is already signs that AT&T will not be as friendly to customers as T-Mobile. T-Mobile will for example unlock your phone so you could use it on another carrier. AT&T won't. Android phones are restricted to the Android Market, and usually not as good line of phones. Also, at least from my personal experience, customer service is better on T-Mobile.
And even if the acquisition doesn't go through and T-Mobile receives the $6 Billion in cash/resources as a break up fee, T-Mobile will still probably have to be bought. The parent company simply doesn't want to keep it.