We typically advocate that competition is good, except when it is not, for example in the case of large production fix costs. Such natural monopolies then need to be regulated. Part of the debate on health care in the United States is also about competition: if health insurance is provided by a single entity, it got to be less efficient. Well, there is data that can verify this, by looking at employers that offer a choice of providers and those that do not.
Ilya Rahkovsky does this and comes to the stunning conclusion that insurance providers that have a exclusivity contract with an employer charge about 40% for the same "insurance quality units." How could this be? Exclusive providers tend to provide better quality insurance because they can subsidize it with the premiums of low quality policies. That would not be possible if they were to compete with other providers.
That said, insurance providers must have been in competition in order to obtain the exclusivity contract, so it is not quite true to state that the monopoly is welfare improving. But from the employees' perspective, it looks like a regulated monopoly in the sense that the employer can keep a leash on the insurance company by threatening to change providers, and that keeps the monopolist from exploiting all rents, it even encourages it to show goodwill to keep the contract. With multiple providers, everyone goes for the quick buck and offers lowly policies.