Dividers are, IMO, absolutely necessary when hauling strange horses -- whether strange to you, or strange to each other. Keeps them focused more on themselves rather than their neighbour that they may not get along with, or want to get along with too well. Sometimes, dividers can give the horses something to lean into a bit as well on corners, acceleration and deceleration.
I'd use dividers because despite being in a trailer, the pecking order still exists. It's too easy for horses to get fighting, especially one trying to trap another and kick the tar out of it, if they aren't tied and separated with dividers.
If your horses are OK traveling together then I don't see a reason to have one. I see plenty of stock trailers with no dividers and 2-4 (depending on trailer size) horses just tied next to each other inside
Agree with the other posters. I for one, have them and use them. I should have clarified. In the stock combo I had it was small and I could put three horses in there if one was smaller. I would only haul my horses, NEVER attempt to haul strange horses together without dividers. I traded it in on the trailer I have now. Just felt better about having dividers.
Many people haul in an open stock type trailer with no dividers, I personally would not haul my horse without them as I want him to be able to lean and use them for balance. In fact when I buy a trailer here in the US, I will adapt it so it is more like the trailers I am used to in England - I will make the dividers go to the ground so a hoof cannot slip under them.
I don't really know the arguments for not having dividers as I don't choose to do that.