We can't argue that after 1 time use the wear and tear of the bristles are that obvious that they won't be as effective. However the amount of bacteria naturally residing in your mouth, combined with residual toothpaste and moisture make your bristle very unhygienic even after a one time use. Count on average about 1 million bacteria added every day to your bristles, after 3 months that's about 100 million bacteria going in your mouth.

According to a toothbrush contamination study from the US National Library of Medicine

Immediately after brushing, the toothbrushes harbored a significant number of microorganisms with no difference between the types of toothbrushes. The antibacterial toothbrush did not limit bacterial contamination.