DaKini I'm assuming you mean why should someone not own DBC. The reason for it is that the way the ETFs are structured it makes them very poor investments. Only suitable for trading. Because a lot of the commodity contracts are in contango currently, it means the ETFs have to keep rolling over the contracts into more expensive ones each month/quarter, continually losing money even if the spot price remains unchanged. Unless you plan on buying the specific commodities and storing it yourself, any holding periods more than a few days/weeks is asking for trouble, if your vehicle is DBC,DJP,GSG,USCI,RJI,USO,OIL,UNG,DBO,USL etc etc.
ETFs that don't suffer from this may be GLD or SLV, but that's because they claim to physically buy the gold or silver and store it.
There's not much contango in currency ETFs which is why those are ok to hold, though they usually don't move that much either way. It doesn't cost hardly anything to 'store' currencies and stock/bond futures, so that's why. It costs a ton to store physical commodities like corn and oil and gas, so contango in those can be significant. Think of it as selling an oil contract to someone 1 year in the future and you store it at your facility. If supply and demand are identical 1 year from now, you're gonna sell it at today's price + your storage costs. So, the buyer is almost always paying up over today's price.