By the time you get to 100K miles, those well worn plugs have put an excessive load on a set of plug wires, and the rest of the ignition system, increasing the chance of failures, and shortening their useful life. A good set of plug wires is between $50 and $120. One coil pack that feeds either 1 or 2 cylinders is at least $50. A cap and rotor for a system that uses the "distributor" to do nothing more than switch secondary voltage from one cylinder to another when the coil fires is at least $50. After about 50K miles, the loss in fuel efficiency from relatively worn plugs is between 1% and 2%, with gasoline over $3.00 a gallon in most places, and the efficiency loss only gets worse.
Tell me again how expensive a $6 spark plug is.
There are two reasons to claim 100K mile tune up intervals. One is to satisfy the EPA. The other is to sell vehicles to people too stupid, too cheap, and too lazy to have them serviced at decent intervals.