Today, many researches on hearing has revealed the importance of having two ears for hearing. It is not just for echolocation, but for hearing different set of sounds and more. One research revealed that that left ear listens for the voices (phonemes and language) and the right ear listens for the musical tune and emotional tonations... check out these links:
Why two ears are better than one:
Left ear for vocalization, Right ear for musical:
Why there are many patients with one bionic cochlear? Blame it on that old Medicare guideline, which is a bible for all private health insurance policy, too. Medicare policy writers felt that having a single bionic cochlear [unilateral] is sufficient for a patient. So the doctors had no choice and offered only one implant on a patient for each timeframe that Medicare allows. One believed that these Medicare policy writers had no idea how much impact a bionic cochlear can do. Today, there will be lots of push for a re-write of the policy to allow the payment of dual implant, aka "bilateral" implant. Most insurances do offer "bilateral" implant, only if a medical professional makes a good case with promise of a successful outcome.
Sources on Bi-lateral Implants:
Results of Google Search