Is it true a flashing kickout at the bottom edge of the roof/wall joint is used to help keep water from getting behind the stucco or siding.

Absolutely.

Roof to wall flashing,typically step flashing,is a necessity for protection against water and ice.

Ken

Kick out flashing is crucial especially in EIFS and siding (http://robsantana.com:2082/viewer/home%2frobsanta%2fpublic_html%2fimages/MWSvinyl.jpg) applications. The kick out diverts (http://www.hgtvpro.com/hpro/bp_exterior_finishes/article/0,2617,HPRO_20149_4243895,00.html)water away from the wall thus keeping the substrate dry.