1. You should learn from others' mistakes because it's cheaper than learning from your own. (approximation of a famous quote)
2. You should learn from other's successes, because it's cheaper than discovering what works by yourself. (simple variation)
3. Applying mutuality principle: how can we mutually benefit from our mistakes?
4. Why not conduct A/B testing and ad word campaigns together so that lessons learned can benefit the maximum number?
5. How to do this?
a) The easiest way is just to blog about your own findings and read others' blogs. This suffers from the free-rider problem: you end up doing research for others.
b) Another alternative is to form a club where each member spends a certain amount per month on ads, and they share their results.
c) Another way is to simply do a lot of Google searches (automatically?) and see what ads are displayed. You're letting Google and the free market do your work for you. This is the e-version of collecting junk mail. You're not being a free rider in this case because your results are being used by others too.