Prepaid legal plans promise to provide a lawyer’s help when you need it for a modest subscription fee of $20 to $30 per month (more for businesses). That sounds like a great way to protect yourself against the risk of hefty legal bills—and more than 70 million Americans have signed up. But when these plan participants later require a lawyer, they often discover the hard way that they aren’t ­really shielded from steep legal fees at all.

The law firms that participate in prepaid legal plans do so to bring clients in the door—clients who can later be billed for services not covered by their plans. Read the small print of a prepaid legal contract, and you likely will find that anything beyond very basic legal services is excluded.

Examples: Drafting a basic will often is included, but not drafting other estate-planning documents such as a trust or guardianship for minor or disabled beneficiaries. Fighting a basic traffic ticket typically is included, but not contesting a DUI. Sending a letter to a creditor often is included, but not handling a bankruptcy. Uncontested divorces may be included, but not ­divorces in which the other spouse hires an attorney.

And while the marketing materials of these plans often trumpet the relatively generous number of trial hours included with the coverage, they generally gloss over the fact that very little pretrial preparation time tends to be included. Pretrial prep almost always adds up to many more billable hours than court time.

Prepaid legal-service plans typically promise a hefty discount on legal services not covered by the monthly fee—perhaps 25% off. But that discount is virtually meaningless. An attorney might apply the discount to a very high billing rate that he doesn’t charge most customers. Or he might bill for additional hours or additional paralegals to make up for income lost to the discount.