Let's assume there is something in the 1st ad text that makes the ad much more highly targets. Like say the heading is "Buy Carpet Now" and first text line is "By Appointment Only"
If the ad appears on the left, the "By Appointment Only" is not going to be in the heading and you can bet you are going to get at least some clicks by people that scanned the page and didn't read the full ad.
If the advertiser wants to maximize conversion rate and maximize ROI on their spend, would it not be a great strategy to pause all keywords where the average position is greater than 3? (I realize sometimes no ads appear in top right or only two ads do.)
Let's assume there is not a huge difference in cost per click between positions 2-3 and positions 4-5.