Personally I think using "${exec_prefix}/libexec" is incorrect. This does not support a multilib configuration where you may have both a 64-bit and 32-bit version of the libexec program on the system. (perhaps this isn't a real problem, but we ran into some conflicts in the past.)
So moving to the libdir, ensures that we are unique per multilib.. and the BPN ensures that we're unique per recipe.