Personally, I leave the divider in, and I have bum BARS instead of chains, as they are more sturdy than chains.
The reason I will only ever use thick steel bum bars is that if I was to get into an accident where the float was to roll or tip over on to it's side, the bars are more likely to hold up without moving, whereas the chains, will either pull tight, or break, which will mean any horse(s) inside will be more prone to further injury, or being squashed by the other horse. A friend of mine had an accident not long ago where the tow vehicle ended up over the edge of a bank, and the float was rolled onto its side with two ponies inside the straight load float. It had bum chains, and as the float rolled over, the chain pulled tight and snapped, letting the divider fall on to the bottom pony with the other one on top.
The other reason I leave the divider in is that my horse anticipates corners... Here in NZ the biggest, or only horse, is loaded in to the right hand side, as that's the side closest to the center of the road. The day we bought Mitch home we opened the (borrowed) 2-horse straight load, to find that the divider wasn't there, we loaded him up anyway and started the drive home, only to find that he knew this amazing game... It was called "corners" and he would see the corner, step to that side of the float, and lean. And when I say lean I don't just mean a little bit, I really do mean LEAN. Like unbalance the float before we get to the corner lean. So he is now condemned to divided floats so he can only lean in one spot