Of course they lose revenue. your view is like any other.
(http://en.wikipedia.org/wiki/Cost_per_impression)
you won't be taking money from the company, which started the ad campaign.
also: don't underestimate the effects on brand value, which will come subconsciously, if you look at ads.
You will know certain companies are still active etc. This is why you don't necessarily need to be targeted. simply informed.
The "sometimes" you talk about should just be with ads on. it won't hurt much, because it's just sometimes.
Only if you view CPI in isolation as money from the sky.
Marketers estimate the CPI they're willing to pay based on estimated conversions per impression. Ultimately, the money comes from sales. It always comes from sales.
If I don't ever purchase anything from a website ad, my impressions are worth zero. My impressions, and impressions by those like me, will drive down the price a company is willing to pay per impression since the conversion ratio also goes down. In fact, my impressions incur a negative effect since it still incurs a cost to the advertiser.
In the best of all possible worlds, only those who will ultimately buy a product are displayed the ad. Resulting in a 100% conversion ratio. Each impression would be maximally valuable and the price paid for that impression would rise to reflect that.
Those of us who use adblock have removed ourself from the market and from the conversion ratio calculations and cost per impression to the advertiser.
They might lose revenue at first, but in a way that is entirely healthy for a larger system of advertisers and clients, in that both advertiser and client reassess the effectiveness of an instrument (an ad network or product), and thus reassess the correctness of a price for an advertising opportunity.
Maybe I'm apathetic to these arguments because I'm quite comfortable paying for content and services, and I'm comfortable with a version of the web where a paying class gets to have paying-class products. If the public feels there should be a free thing, then let it be funded by taxes.
The "sometimes" you talk about should just be with ads on. it won't hurt much, because it's just sometimes.