Many web ads are easily ignored. However, there is a non-trivial percentage of them that are actively hostile - hijacking the browser, planting spyware, playing loud obnoxious sounds, sending flash-driven nonsense spilling all over the screen, and so forth. Web advertiser UGO is notorious for this, for example. So long as advertisers embrace such practices, I will continue to block them. And since web advertising in general doesn’t really work anyway, oh well.