[phonypc]

The benefit of poolish and other styles of pre-ferment is usually attributed to enzymatic breakdown of proteins and starches (especially of starch into dextrins and maltose, which benefits the color of baked bread as well). An overnight ferment probably isn't long enough to get a substantial bacterial population going, hence the multi-day or week process of creating a sourdough starter. Why would the addition of commercial yeast speed the bacteria along?

[lapetitejort]

I'll preface that I'm a beginner and not a food scientist in any right. I tried to make a starter twice and failed both times. I can't say for certain why, but it could have been from impatience not seeing the start rise every day while the colonies built up. If I were to see the starter rise due to yeast action, I would be more likely to maintain it, which would give the bacteria more time to create a colony. So it's not speeding the bacteria along, but rather "faking it until you make it".