While this is somewhat true, it becomes less relevant over time. As transistor size shrinks, leakage and static power loss becomes more and more of an issue.
http://www.ruf.rice.edu/~mobile/elec518/readings/DevicesAndC...
AMULET was an attempt to make a commercial async chip and it flopped pretty early, one of the reasons being interfacing with anything else in the entire world kinda requires a clock.