As with all questions it depends on your overal strategy. If in you market there are significant scale advantages (e.g cola manufacture) by squeezing market share from a competitor you may be able to push them out of the market due to higher costs. Additionally just by fighting hard a company could pull resources away and spend them in an easier market so again you're better off. However in a growing market, or a market where all companies are growing well market share may be less important as there is space for everyone. In fact in some markets companies want to keep some competition so that they don;t get nationalised or investigated by the government for monopolistic behaviour.