Boxer benefits society because he is hard-working and easy-going.  He is sweet, kind, and gentle and the other animals look up to him.  He uses his strength to help others.  Unfortunately, Boxer is a bit too gullible and falls under Napoleon's influence too easily.

Boxer is willing to work toward the goals that his philsophy sets. He is diligent, humble and eager to do good work. These qualities all speak to the benefits he might bring to society. 

The potential harm that a figure like Boxer might bring would come in his inability to think for himself. Though Boxer is admirably adamant in his understanding of animalism, he is not bright enough to articulate his arguments when he sees politics infiltrating his belief system. His brawn is used to acheive ends that his philosophy does not recommend and this is a danger. 

Basically, we need a lot of people like Boxer, but if we have too many, we can get ourselves into a lot of trouble.  If we don't have enough people like Boxer, we get a society where everyone is looking out for themselves and no one sacrifices for the good of others.  But if there are too many Boxers, we get a society of people who never question authority and who can be manipulated to follow destructive leaders.

Boxer represents the kind of person that society could really use a lot of, but unfortunately doesn't have a lot of--hardworking, honest people who do their very best. The average person does not have Boxer's work ethic and concern for the general welfare.

The negative with Boxer is that he just isn't smart enough to see what's going on, and society does have a lot of people like that!

I agree for the most part with the previous response, but I would argue that Orwell wanted us to see that people (or horses) like Boxer are quite dangerous. It is their willingness to acquiesce to power, and unwillingness to question authority, that makes it possible for leaders like Napoleon to seize power. Boxer is physically the most powerful animal on the farm, yet he is almost as unquestioningly obedient as the foolish sheep. That he is also such a sympathetic character is typical of the complexity and nuance of this great book. 

Boxer would benefit society.  His selflessness is one of his most distinctive traits throughout the work.  A tireless worker, Boxer never asks for anything related to self- interest when in comparison to what he gives to the society of the farm.  Boxer's willingness to do for others is evident both in the commitment he has towards his work and how he gives praise to the leadership of the farm.  It is here where I think that Orwell offers a critique of how people must, even at the smallest of levels, ensure that they are not being manipulated by those in the position of power.  Orwell recognizes that because Boxer would benefit society in so many ways, he is easy to control and to abuse.  It is this use that causes Boxer's death in a manner that shows how absolutely cruel the world can be.  Boxer benefits society because of his giving nature and willingness to sacrifice for others.  Yet, Orwell makes it clear that individuals of this nature must be somewhat vigilant to ensure that they are not manipulated by those in the position of power and subjected to abuse because of their uniqueness and selflessness as Boxer was.