Originally Posted by
Bacciagalupe
It can, the problem is that it interferes with the fold on a lot of bikes.
On a separate note, while I vastly prefer drops, I have seen guys with flat bars keep up or even smoke riders on drops. If the rider's position is correct, I don't think flat bars slow you down, other than any discomfort from having fewer hand positions. But if you can't adjust the reach, you might have issues getting the position right.
I had a flat bar roadie for a while and found it was pretty much the same as drops when riding with my fast Saturday morning bunch, except for the sprints and attacks on hill climbs. I felt I couldn't get the "pull" on the flat bars like I can with drops. While I could keep up no problem, it just wasn't the same.
Then again, with the flexible nature of some brands of folder's head stems, would you want to risk pulling too hard by trying to go fast?