Yes, advertising is morally justifiable as long as there is choice to not be exposed to that advertising. If there is a website that you are required to go to for say the IRS or other government services. Or you're required to go there for your school or some other "required" website, then it gets far more murky. But if you are going to a commercial or entertainment or even a news site, then it is totally morally justifiable, since there is no requirement that you visit that site. You are agreeing to the consumption of the content for "free," you are really paying with your attention, and there's nothing wrong with that.
Then when you throw in AdBlock and it's ilk into the mix, which allows you to bypass the attention sale, I think it's totally justifiable.
To take it a step further, if you could somehow mandate (haha) that advertising be easily blockable, then it goes even further into the justifiable category, since only those wish to see the advertising would be seeing it. That's the choice... We should not limit people in what they can and can't do just because we don't agree with where we "spend" out attention. Not that anyone is suggesting that. I don't think the question is whether it's morally justifiable or not, since advertising really doesn't have a moral component, so long as there is choice (and there currently is) - if and when the advertising crosses over into the forced and unavoidable advertising, then it absolutely is NOT justifiable under any circumstance.
I really think that is the ultimate crux here: If a person can avoid the advertising (either through a switch, through AdBlock et al or by not visiting the site) then it's totally justifiable. If it is forced upon the person or on a site that you are required to visit for something that is unavoidable (Government services, etc...) then no, it's not justifiable at all.
Other than that, the free market should decide. If the advertising is too much on a site, then don't visit it... that company will either change it's ways or go out of business.