I've been reading my Coase lately.
Agency costs are sometimes raised as a limit to the size of a firm; as a firm gets larger, agency costs get worse, and for small firms agency costs are tolerated to realize the benefits of lower transaction costs of other natures. I wonder, though, to what extent agency costs could be a reason for a firm, rather than a net cost.
Suppose I'm producing a finished product from an intermediate created by someone else. I'm bad at determining the quality of the intermediate good; if the finished product is shoddy, I don't know whether I screwed up or whether he did. Someone else, though, is pretty good at motivating the other guy, in one way or another, to produce higher quality intermediates. By joining his firm, I'm outsourcing to him the job of handling the agency costs I face in the vertically separated market structure.
Insurance companies that deal with companies often actively help the companies reduce their risks; rather than pay $50,000 to the insurance company for insurance, the firm pays $40,000 and gives the insurance company the authority to inspect the premises, upgrade the sprinklers, and improve the security system. It doesn't make sense for each company to separately involve itself in becoming expert in loss mitigation of this nature, so they outsource it to a firm that is happens to have a great deal of financial interest in loss mitigation both for this company and for others like it. Any company will have to deal with agency costs, though, and it similarly makes sense for people whose expertise lies elsewhere to allow a firm to deal with their agency costs for them.