I think the true purpose of those tabs is merely to lacerate the **BLEEP** out of your fingers.
j/k
They are indeed supposed to:
A) provide tension between the backplane and the case for a tight seal
which leads to
B) helps "seal the box" from EMF leakage
and
c) provides some modicum of grounding for when you're fiddling about back there without bothering to power off!
Asus has the nice non-tabbed "squishy" I/O shields
Cheers