i keep writing little helpers like this in ebuilds:
usex() { use $1 && echo ${2:-yes} || echo ${3:-no} ; }

this is so i can do:
	export some_var=$(usex some_flag)
and get it set to "yes" or "no"

or if i want something a little different, i can do:
	export some_var=$(usex some_flag true false)
	export some_var=$(usex some_flag y n)

useful enough for EAPI ?  or should i just stick it into eutils.eclass ?  OR 
BOTH !?
-mike