and some people may laugh the same way when they see a guy load up 900lbs on a leg press so he can do it one time.

Try curls on it with the ball down / platform up, on one leg and tell me you are not working your core, along with every muscle in your legs right down to your ankles.

It makes a very simple non HR challenging exercise like a curl allot more changeling which is why I do it.

And why not add something that challenges your body a bit more, even if it is only a bit why not, its cumulative so it all adds up, just because it is not burning does not mean you did not work it or benefit from it in some way.

I hate fitness spas with a burning fiery passion.
Why the heck do they exist?
They are useless and their trainers generally are too.

It depends why you're doing the exercise, but I can't see why anyone would do this with a curl. Surely if you're curling then the goal is to train your biceps and not your legs? There's no way you can use the same load one legged on a bosu ball you can from a stable position, so your biceps aren't getting the same quality workout.

Plus if you're doing it to elevate your heart ratee, why on earth are you curling anyway? Turning a curl into a metabolic exercise is a bit like trying to turrn pizza and ice crea into a healthy meal by adding cottage cheese; the improvement is so marginal it's pointless. If you want to train metabolically then train metabolically with demanding movements like squats and deads and cleans. If you want to train for size and strength then do it. Combining both into one performance of one exercise is a pretty bad idea and will give you poor results for both.