I can think of reasons to be honest that have nothing to do with the total amount of honesty in the world, but I don't see any reasons to prevent animal cruelty that are unrelated to the amount of animal cruelty in the world.

Actually I agree. But what we eat does affect the amount of animal cruelty in the world, albeit a very small amount (I should have avoided the term negligible) compared to the sum of animal cruelty, or per capita animal cruelty.

Furthermore, my experienced utility function is quite a bit like yours, where we appear to differ is that I consider mine flawed and I'm working to change it. Therefore I shut up and multiply

I am only mostly vegetarian, and I probably never would have come that far if I didn't fall into a social circle which had a lot of vegetarians. My guess is that the marginal benefit of two randomly selected people cutting half the meat from their diet is slightly greater than one person becoming strictly vegetarian. I think there are many things which are more important than choice of diet, but this does not mean we should ignore the effects of our diet.