There are times when limiting an ad block to only images or only text is beneficial. How it was explained to me by my AdSense rep is that by making one ad block limited, it can create a scarcity of impressions for the other, which drives up the bid price of a click. If the other is at a higher click-thru location, it can improve CPM.
I've done extensive testing on this and I can get my CPM to go up about 30% for about a week, then it goes back to normal. IGWS YMMV.