Heads using rubber hoses like a e46 m3 i assume. In that case there would be plenty of vibration and movement. In our setup its obviously 1 pc so there is no major chance of vibration.
I believe the only reason we have the brackets is to stiffen the plenum so it prevents possible intake leaks at the lower plenum. Keep in mind that the upper plenum is connected to the lower plenum via 8 bolts it does not move much but the lower plenum has that metal rubber gasket that uses spring loaded bolts so it allows some movement when the heads expand when they warm up.
