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Many online references, such as BYO, mention that oxygen is needed by yeast to reproduce, and that a wort with low dissolved oxygen levels will have longer lag times and less attenuation.

This might be a daft question, but why not simply pitch enough viable yeast to begin with so reproduction is not needed?

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Well the reality is that you need some yeast growth to generate the proper flavor profiles with most all ale yeasts. If you underpitch any given ale yeast vs. super overpitching the ester profiles or main flavor characteristics change significantly. There is an interesting line to be drawn between how much growth and how much esters (etc) however.

In the case of lagers I am sure maybe you can pitch without worrying about growth, but I think a little growth helps those strains get going and be super clean, where as weak lager yeast tends to put out a little too much sulphur or doesn't have enough "oompf" to clean up diacetyl etc etc.

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I'm with you on this. For my very first lager, I was really concerned about underpitching. Fortunately, I had a friend working at a brewery bring me about a half cup of creamy white, pure, active lager yeast (S-189) taken from a primary. I stepped it up with a fresh half gallon of wort before pitching. I didn't oxygenate at all beside the usual carboy shaking, and yet I had a full krausen formed on that beer when I awoke 6 hours after pitching.

The problem is that it's just kinda hard to always grow up yeast cultures that big. There's room for infection and error every time you touch yeast (from washing & collecting, to making starters & stepping them up). Its simpler for me to pitch my standard 2 Packets of W-34/70 into my lagers now.

But yeah, I agree with you. If you can consistently build up massive slurries, then you can find other things to worry about besides oxygen.

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The one thing that occurs to me is situations where you want some of the yeast-growth by-products, like fruity esters from a British strain. Pitching enough yeast to consume the entire wort without significant growth is probably grossly over-pitching by traditional standards. The beer will attenuate well, but it may not have the desired profile if much of the character is to come from the yeast. –  Dustin Rasener Feb 21 '12 at 15:24
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