I'm a woman, so maybe my perspective is biased, but I believe the bias against step-through frames has everything to do with gender stereotyping ("that's a girl's bike") and very little to do with functional considerations. My Bike Friday Haul-a-Day is a stepthrough design and bi-partable to boot and I regularly carry myself, my kids (combined 130 pounds), plus cargo and while there's some "flex" it's nothing that's not manageable. My Workcycles Fr8 stepthrough (which was built like a tank) had no flex at all. I also have a flat bar hybrid stepthrough frame and it's no more "flexy" than my husband's equivalent Trek FX 7.2 bike. I use that bike as a "utility" bike as well and the stepthrough certainly makes it easier to mount and handle when loaded with 50 lbs of groceries. It also belies the assumption that a stepthrough frame necessarily requires a less aggressive geometry -- many of the Trek "Stagger" frames have a fairly aggressive geometry. I also have a Brompton, which is a step through design, and while I will concede that has more "flex", that's an inherent part of the design.
All of which is a long-winded way of saying that stepthrough doesn't necessary mean either: (1) flexy or (2) upright and slow.