The Spanish government is spending more money, dedicating more resources, and pissing off more people than they would if they simply let it happen. More people that are quietly and shyly rooting for independence are going to vote for independence because they're pissed off at the Spanish government. The vote would be far closer if they just left it alone, called it invalid, and then dug up some dirt and arrested some leaders on corruption charges or whatever they could get them on down the road, not to mention that they would have to put fewer people, both Catalonian and Spanish, in harm's way and the movement would probably end up dying since the Spanish resistance to it has been its best fuel to this point anyway.