Despite the bright future predicted for the outcome of Sprint Nextel (S) taking over Clearwire (NASDAQ: CLWR), not everybody is cheering the proposed deal on.

A possible stumbling block with the potential for halting the merger is -- paradoxically -- related to Clearwire's clearest asset: its spectrum. Some Clearwire shareholders think the $2.97 per Clearwire share offered by Sprint is too little by far.

In November, when the first rumbles were felt of a newly cash-infused Sprint buying out the almost 50% of Clearwire it didn't already own, one major Clearwire investor expressed displeasure, believing that the company was selling itself too cheaply.

In a letter to Clearwire's board of directors, one also made public in a press release, Mount Kellett Capital Management said it felt "Clearwire's stock to be substantially undervalued." Mount Kellett owns 7.3% of Clearwire's shares.

Mount Kellett felt that instead of selling itself off, it should sell off some of its most valuable asset -- its spectrum -- and use the proceeds to finish building out its 4G LTE network. Selling its excess spectrum could "generate gross proceeds of $6-$9 billion" Mount Kellett estimated.

That price Sprint would be paying for the remainder of Clearwire would be $2.2 billion.

Mount Kellett now has some company in the "don't sell" camp. According to Bloomberg, activist investor Crest Financial said it increased its share in Clearwire from 6.62% to 8.34% to try and block the sale to Sprint.

Sprint has support for the sale from shareholders holding 13% of Clearwire. It needs 24.8% to ensure the deal going through.

Just how valuable is the spectrum held by Clearwire? Whether it's worth what Mount Kellett says or not, it is certainly a prize.

According to Recon Analytics founder Roger Entner, adding Clearwire's spectrum to its own would make Sprint the largest U.S. spectrum holder. Out of the 547 MHz of useable mobile broadband spectrum available, Sprint would hold 200 MHz, as much as Verizon (VZ 0.88%) and AT&T (T 1.30%) combined.

And that raises another possible problem for Sprint. Even if the opposing shareholders can't garner enough shares to stop the buyout, would the FCC allow such a concentration of spectrum in the hands of just one entity?