For the whole "stress on the dropouts" argument. It's bull.

Simply put, F=ma and you can accelerate faster with a lower gear, your mass isn't changed, and friction etc. stays the same from a fixed to geared bike, so only your acceleration is changing, therefore a greater force.

More scientifically, we get into torques which involve the length of the cranks, the force exerted by a track rider compared to a road rider, the difference in bikes mass etc. It gets complicated, but any way you put it, there is more force with a smaller gear ratio. Why do you think cars break their differentials when gunning it off a line because of too much torque... low end torque that fixed gears don't have.