Looking at my semi trailing arms, I can see just what you mean. But wouldn't the angle changes be good in hard cornering? I can see where the angle would be bad if there is heavy weight in the trunk though!
I think multilink makes the tire to road angle more ideal than semi trailing arms can when there is a load in the car. Unloaded, they may be more similar. Think about the semi trailing arms already deflected by weight in the trunk and how that being removed allows better angles in track driving