WebsterBikeMan

The main reason for a drag brake in addition to rim brakes is to dump energy without overheating the rim. A drum brake has substantial thermal mass and cooling fins, and relatively heat-insensitive shoe(s). A disk, on the other hand, succeeds in not overheating the rim, but risks overheating the disk itself, as well as the caliper mechanism. The results of overheating a disk brake, depending on manufacture, include a warped rotor and molten plastic parts, to the point of no longer functioning as a brake (even after cooling).

As has been pointed out, with due care, it can be used as a supplemental brake, but is not appropriate as a drag brake where it is constantly on enough to slow but not stop the bike for minutes on end.

To the original question, Kay and Rudy, who've ridden without one for 35 years, are a light team. I've never used one either, but I do own one, and plan to use it for loaded touring in mountains. We had one descent this summer where I'd have just as soon had it mounted. No curves, just several hundred feet of drop at double digit percent grade, with a stop sign at the bottom. Did not blow any tires, but sizzling hot rims at the stop sign.

We're a mid-weight team, so I would only use a drag brake with a load, as kinetic energy (and hence heat to dissipate) is proportional to mass. And gravitational potential energy, which gets converted to kinetic as you descend is proportional to distance to descend. Which is why it depends on the type of riding and team weight. The other piece - stoker comfort level - has to do with what happens when you hit a curve at substantial speed since you couldn't dump enough kinetic energy without overheating as you approach.