It has been a while since I have worked on an L67, but I'm pretty sure the part you are referring to is the same as the L36. If so, when you look at it closely, you will see that the cover is more than a plate. It is a convex casting viewed from the outside that forms a coolant passage. That cover allows coolant to flow from the interior chamber of the LIM, to the thermostat housing and thence out of the motor. There is actually more than just coolant underneath it, too. When it is removed, you will see two distinct coolant chambers divided vertically by the alumimum wall that isolates the thermostat housing. When a modification is made to block the coolant flow from the LIM through the throttle body, the passage created by the little cover becomes the sole path for coolant to travel out of the engine to the radiator.
A paper gasket with silicone beads for the cover is included in most UIM kits. I don't doubt that they can leak, but it is much more common for coolant to leak onto the cover from leaking gaskets above. This would give the appearance of a leaking cover, when, in fact, it was tight.