Sub-Launched SAMs

"Once more, developers are working on weapons that
enable submerged submarines to attack aircraft overhead.
There was recent successful test of the U.S. Tomahawk
Capsule Launching System (TCLS) releasing a AIM-9X
Sidewinder air-to-air heat seeking missile. This is all part
of an effort that began during the Cold War, particularly for
non-nuclear subs. While most of this work halted when the
Cold War ended in 1991, it has since been resumed.

Last year, for example, Germany successfully tested
launching anti-aircraft missile from a submerged submarine
(U-33, a Type 212 equipped with Air Independent Propulsion).
The IDAS (Interactive Defense and Attack system for
Submarines) missile used is 7.6 feet long, 180mm in diameter
and weighs 260 pounds. It has a 29 pound warhead and a
range of at least 15 kilometers. The main targets are ASW
(Anti-Submarine) helicopters and low flying ASW aircraft."

See:

http://www.strategypage.com/htmw/hts.../20090917.aspx

I always thought sub-launched SAM's were a bad idea, since they
give away the position of the launching sub. But the idea refuses
to die.

Why?