If we think about large advertisers - advertising over a billion impressions per month, optimization would be very hard or next to impossible. Possibly, there would be over 1,000 live placements and if there is more than 1 creative...GOOD LUCK with that.
But thanks to DART's DoubleClick Creative Optimizer you can set the the metric you want to optimize on - Click-through rate, conversion rate, revenue etc. What the optimizer does is that it dynamically moves the impressions to the creative which is delivering higher click-through rate, conversion rate or revenue...and the great thing about it is that it works at placement level. So, if on one particular placement creative A is performing better - it will get more impressions and if on another placement creative B is performing better - it will get higher impressions. One more good thing about this is that you can set a minimal threshold, example serve a minimum of 10% of total impression for each creative.
However, there are some drawbacks with every good system, one of the biggest that I have noticed is that you cannot start with a set creative split. It always starts with an even split. So, if you have 2 creatives it starts with a 50/50 split.
But hey, optimizing at creative and placement level and that too AUTOMATED, I will take it any day.