You have been challenged and you are wrong.
Without speculation, i.e. traders (most done through funds and proprietary trading), you will not have a free market. You cannot always guarantee a buyer and a seller argeeing at a given point in time at a specific price, which will cause market shocks, inability to budget for given costs and hence no way to forecast profit. At the same time, you will get great companies going bust and many others that do not begin in the first place due to unknown risk.
There is a reason there are locals in markets, nearly all day traders.
Of course, speculation can go over the top at times, but to say it does not add a large amount of value, makes you completely wrong.
I disagree with the need for traders, if defined as speculators.
If you look at the foreign exchange market for Australia (something I know about, having run the accounting function for one of the majors), the major banks act as intermediaries. There are exporters wanting to sell foreign currencies and buy Aussie and there are importers wanting to buy foreign currency and sell Aussie - this can be spot and/or forward. The reality is that the exporter, say BHP, does not want to trade currencies with the importer, say Harvey Norman. There are credit issues, tenor differences and volume differences. The major banks by acting as intermediaries resolve all these issues. The majors employ traders to manage the positions created by all these deals. However, the reality is that although there is a large volume of deals, the net position run is always relatively small to avoid major losses to the bank (note by small, I mean that daily VAR for the whole trading floor might be $20m). The bank makes money by putting a margin on the exporter deal and the importer deal (known as sales or enhancement income). The bank also receives speculative/hedging flows from other banks and hedge funds which it manages as part of its overall position. Thus, there is no need for the majors to run significant FX risk to make money due to margins earnt on the deals. However, most majors recognise that seeing all the daily flows gives them a competitive advantage and thus the ability to make trading income as well as sales income.
So yes intermediaries are important, but speculators, I don't think so