If the mixture model frequently choses to use more than one effective component, and if doing so results in a better fit for the held-out data than the unimodal Gaussian, we interpret this as evidence that, for many sentence pairs, human judgments exhibit reproducibly multimodal distributions.
for such sentence pairs, the current practice of aggregating human judgments into a single label would fail to accurately capture the types of semantic inferences that humans might make about the given p/h pair.