When you are training and you start to fatigue, do you push through with poor technique, or do you terminate the set? If the set calls for 10 reps, do you do 10 reps come hell or high water? Do you have some guidelines for what constitutes an "acceptable breakdown of form"? When and how do you make the decision that enough is enough?
It's almost cliche to say that "practice makes permanent", but it is true. If you regularly practice poor form under stress, eventually your mind and body will come to anticipate fatigue and prepare itself to prematurely dole out the slop that it has become accustomed to under those conditions.
Understand that we become sloppy because it is easier to be sloppy. Proper form may use bigger muscle groups, effectively "spreading the load". As we fatigue, major muscles may be activated less to conserve energy, and more stress may be placed on "stabilizers", bones, cartilage, tendons, ligaments - all in an effort to give rest to the prime movers.
Also understand that training with exercises that put you in compromised positions is not the same as arriving at compromised positions because you've fatigued to the point of technical breakdown. Training with an Atlas stone, or doing deadlifts from a sticking point is quite different than doing a set of squats to failure with 25% of the reps looking like a very "bad morning".
If you practice being sloppy when fatigued, you will become better at being sloppy when fatigued. You will make the hard stuff harder.
Training should be PURPOSEFUL. Is your purpose to fatigue yourself, or is it to be stronger?
I'm not going to go so far as to say you should NEVER push to the point of technique deformation but, if you view training as practice (and not simply exercise), then isn't it important to practice proper technique when fatigued? (Depending on your goals) isn't it MORE important to practice good form when fatigued? If you can no longer practice good form, it might be time to hit the showers or, at the very least, call it a set.