In the world of bicycle messengers, using a hi-ten (or even MTB) frame is not unusual. Less likely to suffer damage from repeated abuse - potholes and the like. While it's a little heavier than cromoly, less likelihood of downtime from frame damage, which means less worry of not having a paycheck, a fair trade-off.

I wonder if Brommies fall into the same category. A folder has additional stress to deal with - frame stress around the folding bits as the bike is being ridden as well as the stress of being folded/unfolded. So in a sense, it's over-engineering the frame at the cost of a little more weight. You might have to push a little harder from the extra weight, but it's well worth having your Brommie frame intact at the end of the day.