Some machines must trust one another in order to traffic information between themselves. Your job as a system administrator is to determine ways in which to create the fewest trust relationships possible. In this way, you build a framework that can tell you when a sniffer is placed, where it is, and who could have placed it. The problem is, in most offices, there is no real level of trust. The average American business is in the business of making money.
Tech support is expensive and so is the downtime to restring a network.
Additionally, there can be serious costs involved in that restringing. These are all issues that you must consider. In legacy networks, these are real problems. Also, you must consider the level of risk. These are the real issues.