Not sure if they are absolutely necessary. We installed my wife's ESV rack without using the center pieces and it holds secure just fine, doesn't budge, doesn't shimmy, and holds up at 90mph just fine, lol.
We will get those center nuts tho, but just by utilizing the two stock bolts on the front and back of the burban pieces, it holds.
Let me clarify. Your rack will be secured to the roof without these nuts such that it's not going to blow off the roof or anything like that. But, if you actually use the rack to mount something on the roof, or you use if to pull yourself up on the tire or running boards when, say, washing the roof, it will flex there.
The way the nuts work is like this: there's an eye-shaped opening on the bottom of the rails at the center, and the rail is set down onto the roof over top of this nut. Then, with the rail set down, the rail is slid backward (or forward -- don't remember which) so that the flange on the nut locks the bottom of the rail into place (picture sliding the claw of a hammer under the head of a nail). The center of the rail is now held securely -- and then you bolt down the ends of the rail where the end caps go.
Without this nut, there's no way to secure the center of the rails. There's no access at the center of the rails to the mounting screw(s) like there is before the end caps go on. I think that this center nut IS absolutely necessary.
I did not "pass" on it. Timing was wrong due to circumstances out of my control 
