Practice with a partner that you are compatible with. Nothing much can be learned by practicing with an opponent who is out to prove that he is superior to you. The sets teach you correct limb positioning. They are the ideal angles for dealing with force, for obtaining maximum stickiness, stability and suppleness. The closer you come to keeping this form in actual combat, the better your skill will become.
Sloppiness often results in structural defects which skillful opponents can take advantage of.
Sloppiness is often laziness. Learning Wing Chun as an art does not make it less effective. One of the most dramatic ways to improve the sticking hands training is by keeping a proper stance. When you are in a proper sunken stance with the pelvis tilted forward, your base will be very solid, and your upper body will be very supple. If you have to adjust your stance constantly while fighting, your actions will be too slow.