The discriminative capability of higher-order models suffers too much from the smoothing, since the number of unknown n-grams increases proportionally to the model order.
we assume that at beginner or intermediate level, this facilitating effect is likely to be counterbalanced by the fact that the MWEs encountered are (1) mostly unknown to readers and (2) even more difficult to elucidate using the context as their meaning can be non-compositional.