Well (c) Glencore may be diversified but the market sees it as a natural physical long position, and is treating it thus; and (d) whatever else, some of the firms we know about (i.e. the publicly held ones) look pretty under-capitalised - don't they ?! Actually, very under-capitalised.
The oldest problem. And that takes us back to Enron.
Now look at the percentage of the FTSE that is represented by commodities firms of one sort or another (and see Stephen L's comment on pensions in the earlier post linked above). Systemic risk? O-oh yes.