The ads usually intend to add value. They just often aren't very good.
I don't think advertising is inherently evil (though I respect that some may disagree), I think the problem is that we're still in the early days of Internet advertising. The technique isn't quite there yet.
We could categorize ads into value-added, passive ads versus brute-force, active ads. Passive ads don't only push a product, but provide some useful content for the user, be it music discovery or news aggregation (I guess calling it intellectual stimulation would be reaching, though). The purpose of active ads is to simply push a product and entice the user to buy now. As such, they vie for attention and usually interrupt the user's workflow.
Examples of passive ads: music discovery [pandora], news aggregation [HN], etc versus active ads: traditional ads (TV/radio/internet/magazine) and active product-pushing.
As an aside, I am aware that Pandora has ads which is kind of mind boggling - they are actually providing free advertising to the music companies by letting me discover new music, yet they still have to pay royalties, so they are forced to use ads.
Maybe in the advertiers' and their bosses' deluded minds.
I'd estimate that 90% of ads I saw were lying either directly or by omission. They don't add value, they try to trick you in to buying crap you don't need, that's often no better (or even worse) than the competition, products that can even be harmful (like various prescription drugs that you'd often be better off not taking in the first place).
The people that peddle this crap are slime. But I'm not at all surprised that a large contingent of HN members would defend them and try to justify their miserable, lying, parasitic existence. After all, many of them own companies which depend on advertising. So they try to apply some moral balm and pretend that they're "adding value".
What is actually more offensive to me is the tracking. Ads, I can choose to ignore them. But I don't like the idea of being tracked, and having my every move online watched under a microscope in the name of selling me a targeted product. That's why I make every effort to block ads and scripts.
However, I don't think it's fair to say that "HN members defend them" since (as of this comment), there is a 2:1 ratio of blockers vs. non-blockers. OTOH, this survey doesn't distinguish users who block ads AND use them on their own sites.
I don't think advertising is inherently evil (though I respect that some may disagree), I think the problem is that we're still in the early days of Internet advertising. The technique isn't quite there yet.