It is important to Ponyboy that he participate in the rumble, even though he does not like to fight, because he wants to show solidarity with his friends and also because he is fed up. He realizes that his friends need him at the rumble. Even if the rumble does not change anything in their social interaction with the Socs, Ponyboy needs to show up for his friends and not leave them even shorter on manpower than they would be without him.
It is an unwritten rule among them that they stick together. Two-bit and Johnny even explain this to Cherry and Marcia at the movies. When Dally approaches the group at the movies, he mouths off to the two girls. Johnny tells him to leave the girls alone, but then tells the girls that Dally is not as bad as he seems. Moreover, Johnny says, greasers stick together:
You take up for your buddies, no matter what they do. When you're a gang, you stick up for the members.
Two-bit says something similar,
Us greasers usually stick together...Our one rule, besides Stick together, is Don't get caught...If we needed them tomorrow they'd show.
Ponyboy needs to show his friends and the Socs that the greasers stand together. Ponyboy also knows first-hand that the Socs can take advantage of greasers and he has had enough. After seeing Johnny get jumped, being jumped himself and nearly being drowned by a Soc, and knowing what has happened to Dally and Johnny, Ponyboy also has had enough.