If you have multiple sites, you think that placing adsense in these sites will result to higher adsense revenue. Such that:
multiple sites + adsense= $$$ more money
That holds true, only when all of your sites are performing well, in terms of traffic, hits per day, clicks and impressions. But if one of your sites is performing poorly, google's smart pricing can actually have an impact on all of your other sites- dragging down the performance of your other sites.
The Problogger has a detailed article about this.