Using several of them in parallel helps to validate the individual scores and thus increase confidence in the results.
while they are very useful for indicating quality trends (for example system B with a BLEU of 79 can quite safely be expected to be better than old system A with a BLEU of 60), their interpretation remains cryptic to many users: "What does it mean if my English to Polish MT system gets a BLEU of 50, is this good or bad?