If you look at the life cycle of yeast during fermentation you'll see that all the growth happens at the beginning (lag phase -> exponential growth -> anaerobic fermentation). The thinking is that letting the starter ferment all the way just drains the yeast of their energy and stressed them with alcohol.
This approach definitely works well for me.
For 5-6 gal batch:
500mL starter wort (1.030-1.040) with nutrient. Add yeast, aerate on a stir plate for 4 hours, and then pitch.
Bonus is that I'm not pitching oxidized beer into my wort.