JM: The Aesir are fighting in the hopes of reducing the number of Fenris Brood and Heloki. As well as having a chance at removing the Jotuns. This would make things easier for the group and is well worth it.
There are other reasons but that is the main one.
Remember there is also just one ship off the planet, and the Asgardried cannot hold all the Aesir.
The Aesir are doomed one way or another. This battle gives them the best chance to do the most damage.
"Dulce bellum inexpertis."
Official Forum Expert on Norse Mythology
As Odin says in the Hovamal:
"Praise no day 'til evening; no wife 'til on her pyre; no sword 'til tested;
no maid 'til bedded; no ice 'til crossed;
no ale 'til drunk."