Well, take it like this. If we let China claim the
West Philippine Sea. It's like saying, "the Philippines is just all talk and should not be taken seriously". Also, this will be a big blow to our sovereignty and dignity as a nation. Look at Vietnam, knowing what they are, they're still fighting for it. Paracels are rightfully for Vietnam and Scarborough Shoal is for the Philippines. It is what the UNCLOS dictates. China's historical based claim is weak, we all know that.
Don't worry, China is also feeling the pressure. Vietnam brings along Uncle Russia in it's back and the Philippines has Uncle Sam, Uncle Hu wouldn't dare face a proxy war with Russia and the USA.
China must admit it, if ever they attack the Philippines, the U.S will '
definitely' intervene as part of the 1951 MDT with PHL. Reality wise, in conventional warfare, if China faces off with the U.S in an open war,
America can obliterate their army in a fast quick blow. China is greatly outmatched by both Russia and even just the United States alone.
And China is also afraid of the backlash, a war will do on their image as a major power in the world. No nation will trust China if they just simply attack a small helpless nation like PHL just for such a 'redundant' reason.