Logic!
If you're not resting and fuelling enough to mentally make it through a workout at full intensity, how can you expect your muscles be physically operating at their full intensity and recovering too? A big dose of caffeine might make your mind focus, but it won't fix them too.
Now I haven't tested it, but empirically there's a bit of a correlation between people using pre workouts and low development/progression in the gym...it's usually the bandaid mentality coming through from those who aren't eating and sleeping right to see the development they want. They turn to quick fixes in the first place because unhappy with their training based on adequate food and sleep alone.
The mind is very connected to the body like that, barring glandular issues, it will tell you what it needs and when it needs things. Tired = eat and sleep more. Don't trick it with a dose of caffeine, work with it to fix the underlying issue. Eat more, sleep more.
Once in a while is okay, but routinely taking PWO, you got problems elsewhere.