Considering the CFTC's policy on position limits, there are actually two questions that need to be answered.
The first is whether it is true that speculative positions affect prices. There are good economic reasons to reject this notion. A futures position is a commitment to deliver a commodity by the short side and a commitment to take delivery of that commodity by the long side. Those commitments are distinct from demand or supply of the product. This is because the long side can immediately sell the commodity received to someone who wants to consume it. Likewise the short side can buy the commodity immediately before delivering it per their contract. For the long side, these constitute transfers of the commodity as opposed to demands for the purpose of consuming it. Likewise, generally the short side satisfies its delivery requirement via a transfer rather than through engaging in its actual production. Examinations of the effect that undertaking positions has on prices have consistently found no relationship between position changes and subsequent price changes. This is precisely the result that the above economic reasoning predicts. That is, unless long positions actually shift commodity demanded or short positions actually shift commodity supplied, then trading in futures should have no effect on pricing fundamentals.