Let's suppose that we find out that X is impossible. Good; no one wanted it anyway, so this shouldn't change anything, right? But if you exclude X and set Y as the 0 value for each utility function, then we get u(Y)=0, u(Z)=2, v(Y)=0, v(Z)=-2, (u+v)(Y)=0, (u+v)(Z)=0. The relative values of Y and Z in our preference aggregator changed even though all we did was exclude an option that everyone already agreed we should avoid.

this isn't good for my insomnia