Trouble arises when the whiskers bridge separate parts of increasingly miniaturized circuit boards. They can also flake off and interfere with sensitive optics.
Missiles have been damaged, communications satellites silenced and nuclear power plants forced to shut down. Pacemakers, consumer gadgets and even a critical part of a space shuttle have fallen victim to tin whiskers.
By some estimates, the resulting short-circuits have caused as much as $10 billion in damage since they were first noticed in the 1940s.
The amount of lead in solder needed to counteract tin whiskering is very small; but over time, it's bound to leach out of components dumped in landfills. Fear of lead poisoning has led to zero tolerance for it anywhere, including in the minute amounts needed to keep solder from degrading. They should allow lead back into solder, and demand that all electronic components be recycled. Electronic components should be recycled anyway. Between the cadmium, lithium, and other heavy metals in your typical computer, the lead is the safest thing in there. Not to mention the viruses on the hard drive.