The problem is volatility. If the amount of people that are speculating >>>> than people willing to sell other things to buy "your coin" in a bad market. You could collapse the currency.
A farmer in ancient Rome did not want to undertake the volatility of grain prices when his crop eventually comes to the market, so what he did is sells his future crop to a fixed current price to someone who wants to undertake the risk of price fluctuations, and this way a speculator got the profit which came out of correctly predicting the future grain prices and farmer got a fixed predictable price.
Same goes with these currencies. A speculator might provide enough liquidity to your crypto, in return he gets a better deal for your coins.