The instant response to this of most geeks is that ads are inherently evil. They represent large corporations who generally have different aims than us geeks (especially the open-source supporters).
As a liberal I have another aversion to advertisements in that they are a way of manipulating one's individual choice (a sacred thing indeed!). There is a vast amount of research done on the psychological effects of advertising which shows that this is true.
Finally, ads are annoying. They take up space on the page, they distract you from the important stuff, and they make the pages longer to load. Lovely Firefox extensions like Ad-Block Plus block the ads for you, which is nice, but has other implications (I'll say more later).
Many of the studies done on advertising are about television and magazine ads. Internet ads are somewhat different. There are many kinds of internet ads. Some of them are bearable, some of them are not. Banner ads are bearable, unless there is more of them than there is content on the site. The really annoying ads are popups, or those ones that won't let you see the content for 15 seconds or so (at least most of them have a "skip" link).
These ads don't have as much of a psychological effect on people, because they don't usually have video. Usually they are just images, and small ones at that (compare this to magazines which have big whole-page ads).
Let us now look at the good aspects of Internet advertising. Following the Google business model, one can let users use a website for free, but charge companies for ad-space on those websites. This is great for the user, since they don't have to pay for things. It's why Google is able to give away things like Gmail and Google Apps for free.
The implications of this is that you will make revenue so long as you have traffic for your website. This means that you don't really have to do anything, except to keep the traffic coming. Sometimes this may take a lot of work, other times not. If it doesn't, it is a great business model in that it allows you either to not have to hire many people to work under you, or have enough time for some good R&D.
On a side note, I saw one site (link not available because I forget what it was called) that completely blocked Firefox because of ad. This is dumb, because an ever-increasing percentage of Internet traffic is now using Firefox. People aren't going to switch to IE to view your site (either they can't, like Mac or Ubuntu users, or they don't want to, like most people who have switched to Firefox), they just won't view your site period. Plus, the majority of Firefox users I've seen don't have Ad-Block plus installed and the ones who do probably wouldn't click on the ads anyway. Programs like Ad-Block plus don't threaten the ad-based business model since most people don't care.