That assumption is now conclusively incorrect.
The genesis of the problem comes from the Illinois Trade Secrets Act, which provides as follows:
...a contractual or other duty to maintain secrecy or limit use of a trade secret shall not be deemed to be void or unenforceable solely for lack of durational or geographical limitation on the duty.
The ITSA passed the General Assembly in 1985, and after that point in time, a smattering of cases dealt with the reasonableness of non-disclosure covenants and seemed to endorse them without much analysis as to scope.
But in 1985, around the time the ITSA went into effect, a case called Cincinnati Tool Steel Co. v. Breed held that durational and geographic limitations are required for a confidentiality agreement. A few subsequent cases followed Cincinnati Tool Steel, but not many and the issue was rarely litigated.
(As a potentially interesting aside, "Cincinnati Tool Steel" was not some company based in Cincinnati, Ohio but simply took its name of the president, Ronald Cincinnati. Pardon me for being somewhat of an onomastic snob, but whenever I think of this case, I cannot help but be reminded of James Spader's portrayal of Robert California in The Office, a fantastically geographic-oriented surname.)
Two recent cases, though, have revitalized Cincinnati Tool Steel. In the Appellate Court of Illinois, the case of AssuredPartners, Inc. v. Schmitt (from October 26) held that an unlimited non-disclosure covenant, both in terms of its duration and in terms of the information that fell within the terms of the covenant, was unenforceable. In particular, the court warned that such a broad covenant drastically limited the employee's ability to work in the relevant industry because it prevented him from using any knowledge he gained while employed with AssuredPartners, despite his prior work experience in the industry and regardless of whether he gained that knowledge because of his employment.
The second case, Fleetwood Packaging v. Hein, comes from the Northern District of Illinois, where Judge Tharp invoked the rationale of Cincinnati Tool Steel to conclude a non-disclosure agreement that lacked a temporal and geographic scope was unenforceable. Responding to the argument concerning the ITSA clause cited above, Judge Tharp found that it only applied to contractual restrictions that limited the use of trade secrets - not confidential information.
The reasons for demanding limitations on non-disclosure covenants are sound and generally hinge on two policy rationales.
First, as AssuredPartners recognizes, the presence of a non-disclosure covenant that is unlimited in scope can have a drastic chilling effect and can raise the possibility that an employer will attempt to bootstrap what should be a narrow restriction into a much broader non-compete that the parties never signed. Just as problematically, the relatively undefined nature of confidential information (which by definition is at least partly subjective) can leave an employee twisting in the wind if she stays in the same industry as her prior employer (which most employees do).
Second, it is awfully difficult (if not impossible) for an employee to know what remains confidential after she has left. Much ephemeral business information can lose value quickly. If corporate strategies shift, then a strategic plan that is a few years old may be of no use to anyone. Only insiders should know what remains confidential. Those who no longer are providing services to a company have no reason to know whether certain information retains some confidentiality. Therefore, it makes sense to require some reasonable limit on scope so that an employee is not left uncertain as to what she can disclose about her past work.
Allowing unlimited agreements to protect trade secrets, while certainly a fine distinction, is consistent with public policy. The challenge, of course, is to parse viable trade secrets (which should be more specific and more obvious) from general confidential information (which likely is categorical and less obvious).