What weight are you thinking of carrying there? No seat, lowers the centre of gravity if carrying something heavy.
I know it's only plastic but I had the converter kit that allows you to put the vario mounting plate there and then the top box itself.
If BMW reckon it'll take the weight of a loaded top box, it should be fine. The plastic is easily marked, though.
Think Cymarc do a luggage plate for there or you could buy the kit(not expensive) and fix a vario plate to it. Takes seconds to remove if you need the seat again.