Are you talking about the abductor and adductor machines at the gym? Or any exercise for the abductor and adductor muscles?
The machines at the gym are considered fairly worthless and potentially detrimental because they're isolating and working a small muscle, while neglecting all the other muscles in your legs, butt and core. When you use one of those machines, you're seated, with back support, and focusing on one small muscle. However, our bodies are designed to work on their feet, functioning as a unit, and without external core support. So you can get strong in one very limited muscle and plane of movement with those machines. But unfortunately, they don't have any practical application for making someone stronger, leaner, smaller, or fitter.
And of course, we all know that we can't spot reduce, so strengthening a muscle on the inside or outside of our thighs won't make that area smaller. It will just make the muscle bigger and stronger under the fat layer.
The reason that those two machines can be detrimental is that someone can, if they put enough weight on the machine, build those inner and outer muscles to the point that they make their thighs larger and somewhat distorted. If you focus on one isolated muscle and work it with heavy weights, the inevitable result is making it grow bigger. How is that going to look in comparison with all the other leg muscles that have been neglected? I think that's the opposite of what most women want!
It's far better to work your legs with compound movements -- that is, movements that work many muscle groups at the same time. Examples would be squats, lunges, deadlifts, and step-ups, with all their variations. Compound movements mimic the way our bodies function in real life -- note that these exercises are done standing and without external core support like you find on a machine. And you'll get far better results by working all the muscles in your legs, rather than one or two of many.
Plus there's a significant cardio/calorie burning component in those compound movements, as anyone who's done a few sets of heavy squats or lunges can attest! The highest heart rate I've ever gotten on my HR monitor was while doing BB squats. I actually was watching sweat drip on the floor around me.
You'll never get a workout like that on a machine!
Check out this sticky for info on the inadequacies of gym machines: Train for functional fitness- great article on why to avoid machines!
Hope that helps!