There's one critical thing to understand about big merger rumors - they usually don't make it past the rumor stage. The idea that T-Mobile's parent Deutsche Telekom might buy Sprint has popped up periodically (most recently in 2008) among stock-market analysts, but it's a horrifyingly bad idea that fortunately never makes it past the idea stage.

The rumor raised its head again today, but the idea hasn't improved in quality at all.

1. The Technologies Don't Match. This is the biggest objection, and it's very real. Sprint right now is struggling with three incompatible radio technologies - CDMA, iDEN and WiMAX. The difficulty of merging the CDMA and iDEN networks may be one of the things that caused Sprint CEO Dan Hesse to publicly regret absorbing Nextel. T-Mobile has no technology overlap with Sprint. They would add two more, incompatible network technologies - GSM/UMTS and their anticipated LTE network. The result would have no economies of scale, merely an economy of confusion.

2. Big Mergers Always Create Indigestion. The track record for large-scale wireless mergers isn't good. The AT&T/Cingular merger caused network problems for years. The Sprint/Nextel merger is widely considered to have been a disaster. Mergers in wireless typically work when it's a much bigger company swallowing a much smaller one - Verizon/Alltel, for instance. In this case, smaller T-Mobile would be trying to absorb much larger Sprint.

3. George W. Bush Isn't President. The Obama administration has said they're going to take a much tougher line on mergers that may reduce competition than the Bush administration did, and merging two of the four national wireless carriers would get the ultimate scrutiny. Since neither T-Mobile nor Sprint are on the verge of failure, it'd be hard to paint this merger as a rescue.

4. There's a lot on T-Mobile's Plate Right Now. T-Mobile's UK division is trying to merge with Orange, another UK carrier. So obviously, what T-Mobile corporate needs is to handle another mega-merger at the same time. That makes perfect sense. Right.

5. Sprint Doesn't Have the Answer for T-Mobile's Woes. Apparently, T-Mobile CEO Rene Olbermann is concerned that T-Mobile is stuck in the lower-cost area of the marketplace. Guess what? So is Sprint. T-Mobile is a metropolitan-focused carrier with shorter-range AWS and PCS spectrum rather than the longer-wave cellular and 700 bands. So is Sprint. T-Mobile is having trouble figuring out how to get their customers to pay more dollars per month, while upstarts like MetroPCS and Cricket cut away at their lower end. Ditto Sprint. In this case, banding together doesn't look like they'd come any closer to solving those problems.

6. T-Mobile Doesn't Need Sprint's Problems. Sprint is still coping with culture and technology clashes in the wake of the Nextel merger. They're recovering from lingering customer service problems, they seem confused about their prepaid strategy, and they have a complex and perplexing relationship with Clearwire and their on-again, off-again cable company partners. They're improving, to be sure, but they still have a road ahead. T-Mobile, which prided itself on running a streamlined business with great customer service, doesn't need that kind of drama.