Curls were designed to work the biceps by extending and contracting them against resistance. So long as weight increases work only the biceps harder through their appropriate range of action, all is good. The harder the involved muscles work, the more they develop (assuming adequate rest and nourishment, of course). Once you apply an excessive amount of weight, however, a shift occurs.
Using much more weight may make you feel as if you’re working your biceps harder, but if you have to cheat, just the opposite is true. As soon as your biceps are totally overwhelmed and need the legs, the back and the front delts to help them heave the bar through a rough approximation of the curl, the biceps are relieved of maximum work’you wear yourself down and get poorer results.
Does that mean you should be wary of weight increases? Yes and no. Yes in the sense that if weight increases become unmanageable, they defeat their own purpose. No in the sense that unless you constantly strive to handle increasingly more resistance, you will not attain maximum development. Even so, you must strive to use more weight sensibly. That’s the key to success.