OK, let's try this again... I just typed it, and my computer crashed on me... figures....
The reasoning for the back panel to be left open is for suspension movement under more extreme conditions. If a "harder-than-usual" bump in the road happens, the suspension would be compressed greater than normal useage would require. In some cases, this could cause the watts linkage to hit the back of the body. If that panel is left "loose", it will simply move out of the way sort of speak. If it is solid to the floor, it will most certainly crack. This would also be the case if, say, you set your suspension up for yourself either alone or with, say, someone under 200 lbs but then you take your 350 lb. buddy for a ride. It would certainly take less of a bump to compress the suspension more at that point.. Please guys, no fat *ss jokes needed here.. LOL