If you pitch enough yeast to reach the saturation point - where the aerobic phase ends and the anaerobic phase begins - I suppose you could omit aeration. But that's kind of stupid, because it's more painstaking work to grow such a large starter than it is to just aerate the wort. Oxygen dissolved in the wort is an essential nutrient for the yeast in the aerobic (reproductive) phase of fermentation.
Moreover, here's a bit of interesting stuff from George Fix:
Oxygen and diacetyl are linked [...]. Hoffmann has shown that inadequate oxygenation of chilled wort can lead to elevated diacetyl levels. In one of his test brews, the dissolved oxygen content of chilled wort was a mere 0.80 mg/L. The second brew had 10 times that amount (8.0 mg/L), which is a widely used value. To achieve this level of oxygenation, one typically must saturate chilled wort with direct oxygen injection. Hoffmann reported that after the seventh day of fermentation, the poorly oxygenated wort had a diacetyl level of 0.80 mg/L, whereas the level in the second brew was down to 0.20 mg/L.*
Keep in mind the detection threshold for diacetyl is 0.10 mg/L. So the lower the oxygenation rate, the greater the amount of diacetyl - a byproduct of fermentation - in the finished beer. In some beers, a significant amount of diacetyl is acceptable; in most, it is not. Further, diacetyl tends to be unstable. It will devolve from the butterscotch flavors to rancid, raunchy flavors over time.
There are other reasons to avoid overpitching. Foremost is that yeast develop the majority of their flavor-precursor esters during the aerobic phase. That's why underpitching is so undesirable: even in a fruity British ale style, underpitching leads to excessive ester production, because the yeast overwork themselves during the aerobic phase, just reproducing enough cells - and if the underpitch is too bad, they'll never reproduce enough - to properly saturate the wort. In the same manner, overpitching is bad: pitch sufficient yeast to skip the aerobic phase, and the yeast will never produce detectable esters. Bad news for English and Belgian ales.
According to White Labs, overpitching can lead to underattenuation.
If you pitch the appropriate amount of yeast into a properly aerated cool wort, your lag time will be within specification and your beer will suffer no indignity. If you take shortcuts hoping to skip steps, you'll only be back here in a few weeks asking us why things went wrong.
It's best just to follow the accepted brewing procedure. It's there for a reason, and you skip steps at your peril.