Except at that rate it's really only worth it if you're putting in over $100k.
I very much doubt many people are turning up with $100k to invest in the "stocks" on MPEX.
Actually you need to trade something like a few ks worth of BTC over your entire account lifetime to recoup the fixed fee out of what you save on % fees. That aside, you're welcome to doubt anything you please, MPEx still trades 1-200k month in, month out. A farmer doubting the Earth's really round, what's that do? Doesn't matter in the slightest, he can go about plowing his field just as well whether he believes the Earth is conical or banana-shaped.