Should I use a router speed-control unit?

I have two routers, a Porter-Cable and a DeWalt, plus a separate speed-control unit. Both companies told me not to use separate speed controls because they'll damage the routers. Why?
—Leonard Ambruso, Los Osos, California

While a rheostat-type speed control may not damage your router's electronics, Leonard, it could stall the router by robbing it of current. A representative for Porter-Cable went a step further, adding that some tool manufacturers will void the warranty of router owners who modify their tools with after-market devices the tool was not designed to accept.

Back when routers were mostly just high-speed motors and a chuck, a simple rheostat-type speed control regulated motor speed, by varying the amount of electrical current. Though imprecise, they allowed you to reduce your router speed to work safely with bits of different diameters, as indicated at bottom.

With today's routers, even single-speed models use electronics for soft starts and to maintain constant speed under load.