Yes, but it;s also a major festival. Lots of civilians from around Remnant, people's guard is lowered because they're busy with the festival itself. Think of how much of a propaganda coup it would be to have a major infiltration of the city by Grimm, during what is one of the biggest festivals of the year, in the full view of the world. What does that say about the Vale government and it's ability to defend itself as well as protect it's citizens?

Basically, even if they had been defeated, it would have been a massive blow to the reputation of Beacon (For a real life example, look at the Sochi Olympics and the Volgograd Bombings), and it gives them legitimacy and public awareness. And fear.