The question is somewhat vague, and depending on the person reading, the responses should vary. Here is mine.
Personally, I feel its a bonus for ANYONE to read philosophy. Not just martial artists. Many of the greatest philosophers in history did not even study combat arts.
It can be very eye opening to see how the greatest recorded thinkers through out history viewed certain aspects of this journey we call 'life'
I'm not just talking about any specific philosophy. Not just eastern, nor western. Not just classical, nor modern. But all.
The insights one can gain by pondering the many aspects of existance, can be life and world changing.
I have ready many thoughts that I have not agreed with, yet that does not mean i wasn't able to glean something usefull from the outlook that particular person had.
It has been said that learning our history is a good thing, because history is doomed to repeat itself. Learn from our mistakes. IMO this doesnt just concern factual data as to what happend when and how, but also to learn the thought patterns and processes in place to see the why of it all.
Using confucius as an excample, you can see the dramatic effect that confucius had on the chinese social structure and development. Much of the reactions and interactions people had and made, namely heads of state and officials, leaders and generals, were tied in one degree or another to the social structure in place during the certain time periods you would be studying. The particular social structure being directly tied to the school of thought developed by the confucians.
For whoso comes amongst many shall one day find that no one man is by so far the mightiest of all.