Unless you are pumping filtered air into the starter vessel while its on the stir plate, you aren't getting a good aeration of the wort during its growth phase. Stirring is very good for improving the transfer of gases from the atmosphere above the wort, into the wort. But if the atmosphere above the wort is depleted of oxygen, all the stirring in the world won't make any difference. During active fermentation, that atmosphere will have little oxygen and a high percentage of CO2. If an Erhlenmyer flask is used as the starter vessel, the avenue for gas transfer is restricted. To keep contaminants out, we typically have some sort of barrier (foil, foam) over the flask mouth. That is an impediment to gas transfer and diffusion. An active flow of filtered air into the flask alleviates this deficiency. There is no need to bubble the air through the wort, just exchange the atmosphere.