I heard it more than once from many people and some teachers as well.
Gym + Dancing = Bad idea cause you might lose flexibility and the ability to use muscles isolatedly.

Though Im curious. If leg , abdomen and lower back muscles serve as the means to make , control and time movement and of course to balance us , would a gym program specifically designed to train those muscles exclusively prove fruitful or would it still be hindrance ?

I'd be interested in hearing information on this also. I can't say I've ever heard of a gym workout hindering your dancing.

Certainly a standard gym workout doesn't challenge you to use your body in the same way, but how can and improving overall strength and endurance be a bad thing?

Personally I prefer pilates/yoga/dance based fitness programs, but still do the more traditional workouts for variety as well.

"but how can and improving overall strength and endurance be a bad thing?"

It can reduce flexibility if taken too far.

Do you mean if I were to try and bulk up? I have heard that excessively large muscles can actually reduce flexibility. But that usually applied more to men/women whose goals are muscular size/body building. How many of us actually go that far, as dancers? I've also read that flexibility without proper strength is harmful as it leads to greater injuries.

It isn't just a question of flexibility. If you bulk up too much it can destroy your endurance.

All those muscles suck up blood, lots and lots of highly oxygenated blood. If you bulk up too much you aren't going to have the stamina to make it through a full dance set.

ALSO...those big heavy muscles can be too much to actually hold up. You would think with the reps and the weights it wouldn't work that way, but you have to realize biceps and triceps, etc. can weigh a ridiculous amount.

If any of you are boxing fans, you will instantly understand what I mean. The guys who get too muscular are in greater danger of punching themselves out because their arms are heavy, and it takes too much energy to swing them and control them. Its counterintuitive but its true, and its the same in dancing. Think of how much of the time you have those muscles engaged and moving, at or close to shoulder height.

In terms of legs, be careful there too, because if you bulk those muscle up too much you won't be able to achieve proper leg positions which can be very important. Think about how much flexibility you have to have in your hips, thighs, knees, calves and ankles to do even simple latin steps.

A little pilates? Yoga or aerobics maybe, but hardcore shaping? Trust me, I have a bunch of muscle heads in my life and it isn't going to help you dance.

Fruitful or hindrance. It would be of benefit to speak with a certified trainer, in the area of goal oriented exercise that your interested in. I do go to the gym. I mix up my programs and do circuit training as well as dance. Cardio is a great benefit for stamina in dance, as well as everyday life. Dance is not my only outlet. I have yet to lose flexibility even with free-weight workouts - then again I'm not a body-builder nor interested in bulking up. Toned and fit is more my goal. Without the assistance of a qualified trainer - at first - I really would have been lost as to how to accomplish fitness goals that could merge into dance as well.

THere is no reason why building muscle tone, endurance and cardio-vascular fitness will bulk up muscles significantly. For muscle bulk we're talking about small reps of heavy weights. Muscle tone and endurance is lower weights, higher reps.
Flexibility and balance can be worked on too. Cardio-vascular fitness has negligible effect on muscle bulk and can only improve dancing of all forms. Good muscles are essential to support a wide range of movements without injury. How can a standard ballroom dancer keep a good frame without strong core muscles? or perform strong, controlled movements throughout a day's competition without muscle endurance?
There is performance-specific fitness of course, but it's based on a good solid foundation.

"How can a standard ballroom dancer keep a good frame without strong core muscles? or perform strong, controlled movements throughout a day's competition without muscle endurance?"

They can't, but that's not the important question. The important question is if the specifically appropriate form of these things are to be gained in a fitness facility or by practicing the actually targeted activity - by dancing.

We might also ask how your primitive ancestors got in shape for (taking a little dramatic license) running down mammoths and running from sabre toothed tigers - probably not in the gym, but first by chasing each other around the campfire as kids, and later by leading the life they needed to be in shape for on a daily basis.

Just one look at the muscle tone at the back of the lady Latin Dancers will show that they do some kind of Gym work. You will find the same with the Standard Style also. In Standard the teacher that I go to attends a Gym three times a week. You've only to put your arm and hand around her to feel the tone of her body. She has a personal trainer for the Gym work.

My wife teaches fitness classes called "Shape and Tone." Those are probably the key words. It is good to tone your muscles and make sure your body is in good shape. That means a nice mix of cardio and strength exercising. It does not mean bulking up with free weights and getting muscle-bound. Dancers are strong, but not disproportionately. I am a boxing fan, as another poster suggests, and I do believe dancers could take a cue from a boxer's training regimen. It is mostly about flexibility, endurance and quick feet and hands. It is not about massive arms and chests. That's for football linemen, not athletes (and dancers) who have to move quickly and precisely, even though in this day and age, the best football linemen are also learning that quick moves with their feet are just as important as bull-like strength.
I am middle-aged soft now, but feel I am in pretty decent shape. I am not "tough-guy" strong with big arm muscles, but I have good endurance and very strong legs and knees for my age. So I now rely heavily on stretching exercises such as yoga and some resistance strength exercises on a machine called Total Gym. That, and a lot of dance practice, I hope will keep me clicking for some time.