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I’ve already taken one crack at explaining why AT&T(s t) would want to buy regional carrier Leap Wireless(s leap). This deal is clearly about spectrum. AT&T wasn’t able to get it by purchasing T-Mobile so it’s hoping to pick up airwaves on a more piecemeal basis by acquiring smaller operators. But how much spectrum will AT&T get and is it worth the extremely high premium AT&T is offering?

Explaining these things is always easier when you can paint a picture. Luckily, the good people at Moasik Solutions, a company that catalogs mobile coverage worldwide, have generated some detailed maps showing the spectrum positions of both operators as well as what the two operators would look like once combined.

Let’s start with Leap, which is known to most of the country by its consumer-facing brand, Cricket Communications. As you can see from the map below, Leap isn’t a nationwide carrier, but it does hold a few key licenses that would make any carrier’s mouth water. It owns valuable spectrum on the dense mid-Atlantic seaboard from Philadelphia down to Richmond, Va. But Leap’s big appeal is west of the Mississippi, where it has 26 MHz or more of PCS, Advanced Wireless Services (AWS) and 700 MHz airwaves — all bands where AT&T wants to build mobile broadband networks — in cities like San Antonio, Houston, Denver, Kansas City, San Diego, Phoenix and Portland, Ore.

Leap also owns smaller chunks of frequencies in Chicago, Pittsburgh, Memphis, Nashville, Charlotte, New Orleans, Oklahoma City and Salt Lake City. AT&T could put all of those airwaves to work in its 3G HSPA+ and 4G LTE networks. Harvesting that spectrum is almost certainly what AT&T plans given Leap’s primary technology CDMA is entirely incompatible with AT&T’s GSM and HSPA systems.

Now here’s another Mosaik map showing AT&T’s spectrum holdings:

As you can see AT&T doesn’t lack for nationwide license coverage (though it hasn’t deployed networks everywhere it has spectrum). In all but the most unpopulated stretches of the American landscape, AT&T holds at least 26 MHz of spectrum, and in densely populated cities that number grows to more than 70 MHz and in many cases over 100 MHz. With more airwaves AT&T has more capacity to build bigger mobile data pipes, and these maps show AT&T is already has quite a lot of capacity. These maps don’t even include the Wireless Communications Services (WCS) airwaves AT&T owns and just got permission to use for LTE.

Here’s Mosaik’s representation of the new spectral landscape if the merger is finalized:

The before and after maps really don’t look that much different right? The after-map definitely gets darker in Leap’s coverage areas, but in most cases those areas were pretty dark to begin with. The point I’m making is AT&T isn’t suddenly going to find itself with a windfall of spectrum in any big cities if this deal closes. You can think of this deal like expensive plastic surgery. AT&T will use Leap’s licenses to surgically enhance its capacity in some areas, but it will still fundamentally be the same operator when the bandages come off.

Here’s another Mosaik map that illustrates that point. It shows everywhere, AT&T (in blue) and Leap (in orange and green) has LTE deployed. Admittedly Leap is just getting starting on its LTE rollout, but the map is still telling. Except for a few towns on the U.S.-Mexico border and a few fingers of coverage outside Tuscon, Ariz., and Houston, there’s nowhere Leap has an LTE network where AT&T doesn’t already have one.

Is this deal really worth the price?

AT&T isn’t just paying $1.2 billion for Leap – a premium nearly double Leap’s closing price Friday – it’s taking on $2.8 billion in Leap debt. That’s $4 billion to acquire regional spectrum, much of which isn’t in areas where AT&T needs the capacity.

To put that in perspective Verizon(s vz) just bought the cable companies’ AWS licenses for $3.9 billion, giving it loads of 4G spectrum in nearly every single major city in the country. Verizon will be able to do a lot more with those cable airwaves than AT&T ever could would with Leap’s spectrum. In fact, Verizon is already using its new frequency windfall to build a second nationwide LTE network, which in many places will have twice as much capacity as its first.

You could easily interpret the Leap offer as bad deal for AT&T, but Ma Bell seems to have concluded that the spectrum market is now so tight that airwaves — so long as they’re in the right band — are worth any price. It’s the same calculation that T-Mobile made(s tmus) when it merged with MetroPCS, paying Metro shareholders $1.5 billion and giving them a quarter of the combined company.

When rumors of both the T-Metro and the AT&T-Leap deals surfaced, I dismissed both of them, writing on GigaOM there were practically no advantages for any of the companies involved. I was clearly wrong on both counts, and I’m left eating crow.

But I’m still shocked that this all happened. Why? Because it demonstrates just how much the economics of the mobile industry has fundamentally changed in a little over a year.

Mobile operators used to buy each other for their subscribers, footprints and networks. Cingular bought AT&T to become the single-largest mobile player in the U.S., combining the two companies GSM operations. Verizon bought fellow CDMA operator Alltel to expand into small markets and rural towns all over the country. The one big case where two operators with incompatible networks merged, Sprint-Nextel(s s), is universally regarded as a failure.

But today mobile carriers are buying up their competitors for a single asset only, spectrum. The big four are becoming chop shops, buying up smaller players and stripping them to get at their airwaves.