Thats true Spork, but adsense allows only three ad zones per page, as does adbrite. However bidvertiser allows you to place as many as you like so you put 3 adsense zones + 3 adbrite zones (with the same sized bidvertiser codes added) and then some bidvertiser codes on each page.
Basically means you get paid from at least one company everytime someone visits your website and also if sombody clicks on an ad.
Its all good, so long as you don't really overdo it LOL
;-)