You shouldn't admit weaknesses, for one when they are asking that, they are already impressed with you so far, that's why you are interviewing and has thought's of hiring you, why take yourself down a level or 2, Interviews are all about selling yourself, not breaking yourself down for no reason. A person saying they have no weaknesses, displays supreme confidence in oneself, but you don't want to be too cocky, that's why I put the "but if I had a area I could improve in".
Would you tell a girl you are trying to fukk that you cheat sometime, or ejaculate prematurely,