We get lots of suggestions about this. There are several reasons we don't allow ads to download and then just hide them. One reason is that downloading ads takes up a LOT of bandwidth. Most people don't have unlimited data plans. Or unlimited time! Compare how fast a web page loads with and without AdBlock. :)
Another reason is that more and more often, ads carry malware the way mosquitoes carry diseases. Think of AdBlock as your online mosquito net.
The most important reason we don't do this, however, is that we promise to block annoying ads. There are probably some extensions out there that will allow you to do this. AdBlock just isn't one of them.