oh, naa dont worry about that! i think is some kind of isolation thing, im not quiet sure, one of the manuals for a head unit i had mentioned it i think, ill have to see if ive still got it, its nothing critical though i know that much. my renault had an open back bit, as do escorts, fitted stereos in both and never had a problem! and mk2s have too by what you said ive fitted one in my mates, his is still good.
edit: you seem to be right, google thinks it adds extra support too, never had a problem though, just go for it!