Why do we study the history of philosophy? Well, there are obviously lots of good reasons. But one important reason is that it gives you perspective on your own philosophical views.
It's very difficult to believe that in the future many will look back on many beliefs that are common today as misguided ad perhaps deeply problematic and wrong. We feel this way now about many historical beliefs, and there's no reason to think that the present is epistemically privileged. This is one reason we have not to be overly concerned by the unpopularity of animal rights theory. We know that many correct theories have historically been very unpopular, and it's easy to see how previous prejudices against correct theories are potentially analogous to prejudices against animal rights theory (hence the discussion of speciesism.)
Of course, this is not an argument that animal rights theory is correct. Many theories (ethical and amoral) have been unpopular when first advanced, and remained so because they were incorrect. I believe that people who believe in moral obligations to plants are greatly mistaken, for example. The point I'm making is just a simple defense of holding (very) unpopular ethical views.
This line of reasoning occurred to me a long while ago, and is perhaps, in part, why I have been willing to accept animal rights theory. Perspective on my place in history made it imperative for me to decide which, if any, of my beliefs might (correctly) be judged harshly by future generations. Of course, animal rights theory could be correct, even if it is never accepted by future generations. I just hope that they'll be more ethically progressed than we are.