I don't like flat bars either. If I were to ride a flat bar I'd get something with a good amount of sweep like the On-One Mary. The ideal bar that achieves the neutral natural hand position is a 3-speed cruiser type bar, but these have limitations. I find that the handling is only good for sitting upright. You can't get aero on these things and standing up while peddling is awkward. Flat handlebars though uncomfortable for me has better all-around handling characteristics vs. the 3-speed bars. Randonneuring road bars is the tops in my book for comfort and multiple hand positions. Moustache bars is a good compromise because you get the neutral 3-speed hand position when held by the ends and a more aggressive road-like position at the bend. The only thing that it lacks is a drop like road bars so you can't get too aero for headwind.

One of the things that's keeping me from getting a folding bike is the fact that most of the lower priced ones come with flat bars and you can't switch them out. I was looking into the Xootr Swift, but the top tube is 56cm which rules out road bars entirely for me. Flat bars with little to no sweep suck.