The merger of Sprint’s and Clearwire’s WiMAX-based mobile broadband businesses is leaving analysts with a nagging question: What do cable operators get for the $1.65 billion they invested in Clearwire?

Clearwire CEO Ben Wolff suggested in a statement Monday that Clear is a rival to next-generation services from AT&T and Verizon Wireless.(Clearwire Closes Deal AmidHurdles) Clearwire’s service, dubbed Clear, competes directly with cable modems, a cable operator cash cow. 

The high-speed service is currently in 50 markets across the UnitedStates, which gives it about a sixth of the reach of either Verizon Wireless or AT&T.

It would take all or more of Clearwire’s $3.2 billion war chest to stretch its still largely untested WiMAX service across the country, according to analyst Tim Farrar, president of Telecom Media and FinanceAssociates.

Clearwire’s current network is based on a proprietary technology developed by Motorola, rather than WiMAX. Sprint’s XOHM network isbased on WiMAX and will eventually account for 70 percent of the merged network.

“It will be a very expensive and a difficult leap for Clearwire to go from a fixed service to the home to a high-value mobile service that competes with Verizon Wireless and AT&T,” Mr. Farrar said.

Cable leader Comcast invested $1.05 billion in Clearwire, while Time Warner Cable plunked down $550 million, and cable operator Bright House invested $100 million. (TechConsortium Bids $14.5B on WiMAX)

Cable operators have long sought to match both AT&T and Verizon with a wireless service to add to their triple play of phone, Internet access, and TV.  

That has proven so far to be elusive and expensive.

A consortium made up of cable companies Comcast, Time WarnerCable, and Cox Communications along with Sprint Nextel spent $2.37 billion on auctioned spectrum in 2006. They have so far done nothing with the spectrum. (Cable Goes on Spectrum Spree) Last year Sprint pulled out of the consortium.

Now the cable operators are spending another $1.65 billion on a wireless service that may never compete head to head with mobile offerings from AT&T and Verizon.

“The cable companies have an array of competitive issues which are more pressing than wireless, like DSL, fiber-optic services, and satellite TV companies,” said Joe Nordgaard, director of wireless consulting firmSpectral Advantage. “Wireless data will give them some opportunity to offer services outside their coverage area, but that will not address their core offerings.”

And in a recession companies are more likely to protect their core than begin marketing me-too alternatives in a market where consumers are counting their pennies.

Cablevision chose a different route. The Bethpage, NewYork, cable operator is spending $300 million to build a Wi-Fi network in its tri-state coverage area. The outdoor service will be free to Cablevision’ssubscribers. (CablevisionUnwraps Free WiFi and CableChallenge: WiFi or WiMAX?)

Perhaps the other cable operators have new integrated services in mind for their Clearwire investment, but as it stands today, the goals of the investment remain confusing.