One of my sites received search engine traffic from 446,328 keywords in the last 30 days. That's a lot of keywords, but there is a more interesting story here...
The top one-hundred of those keywords returned 26% of all AdSense revenue for that site.
The majority of those 100 keywords are the result of keyword research. Without that research, the site would be making only three-quarters as much revenue.
The top 500 keywords are responsible for 43% of all revenue. I'm not suggesting that you ignore the long tail completely, as it does generate a good amount of revenue - but I believe that you should consider that as additional revenue.
If you want to dominate the SERPS, keyword research is a necessary step on your path to victory.