First of all, the use of iridium is not as a coating, but as a solid metal electrode, typically a fine wire type tips. Iridium has properties that make it ideal for use in plugs: Excellent electrical conductivity, incredible heat resistance, and it is very hard physically. Typically, iridium plugs (like Denso) are single electrode designs, as they do not suffer from the issues more common metal electrodes do. These properties make them long lived (100K miles is common), and excellent choices for "plug killer" applications like turbo charged engines.
