This is a question that has probably been asked since Google introduced the feature. When you set up a campaign in Google Adwords, Google by default puts your campaign’s ad-serving to be optimized:

As you see in the above picture, by choosing the “Optimize” option, you allow Google to show your better-performing ads more often. So that’s a good thing right? After all, your better ads keep showing up, and you get better results. Add to that the fact that Google is out to take care of you, and you are golden, right? Wrong! The last time I checked Google was still a public company, out to maximize its advertising revenue. By choosing the optimize option you simply kill any chance that you have for conducting a true A/B testing on your ads. Don’t forget that Google chooses this option by default, so you need to pay extra attention every time  you set up a campaign. You don’t want to spend thousands of dollars and find out that your A/B testing never happened.

Now here is a general rule of thumb: love Google but never trust it. This pretty much applies to all Google dilemmas.