It causes volatility to drop against fiat, yes, but it also causes the volatility to drop against other goods and services of the same variety. If somebody is selling an old laptop, for example, then you can get a feeling of how much an old laptop will go for, even without the fiat price. Now if there are 10 old laptops of different varieties and in different conditions, then you get a spectrum of what old laptops will go for. And you definitely don't need to look at the fiat price to get a feeling of what you should charge. The first person to sell an old laptop directly priced in Monero has to look at the fiat price and then after that people need to look at the fiat price less and less