CTR doesn't differentiate between accidental clicks and clicks from people who appreciated the advert. Sure, you could give it a gut judgement based on industry standard CTRs / personal experience, but it would be very rough.
To actually judge whether users wanted to see the advert the best metric would be conversions, but that's likely data the site owner wouldn't have (and even with that data, you'd need something to judge it against - for all the talk that most adverts are ignored, the fact remains that some adverts through Google/Facebook/whatever ad-network can lead to direct conversions with a positive ROI).
Not saying it wouldn't be interesting to hear what his typical CTRs are, just that nobody should look at the answer and think you can make significant judgements based on them.
It's better than nothing, but it certainly isn't better than rough.
Setting aside what's "average" - are you controlling for the type of advert (both what it's advertising and its format), the type of audience, etc...
Over the years I've seen plenty of examples where CTR and ROI just aren't linked at all. Right now my work doesn't involve a huge amount of this kind of advertising (though I do have experience overseeing 7 figure budgets in the past) so I don't have much great data to pull examples from, but here's one real example from a couple of months ago:
Two not-huge (sub $10k) media buys, both direct with websites (not through networks), both with websites that are well-respected by their readers and by their industry, both using the same adverts. One of the sites excitedly sent us a report about how well the ads had performed, a 5.63% CTR for banner ads, clearly we're popular! The tracking on our side showed that of the just over 15k clicks they sent, 12 converted into users for us. Meanwhile the other site was reporting a 1.32% CTR, sent just over 6k clicks (pricing and order size wasn't exactly identical between the two sites), and landed 700 new users. I know both companies well enough to not suspect click fraud, and I've seen the company that performed badly perform well on other campaigns. But if you just looked at this comparison, there's no argument that the campaign with a much lower CTR was actually hitting an audience that was more interested in what we had to sell than the one with the high CTR. There's lots of possible reasons, from accidental clicks depending on how sites are set up to the audience targeting each site did (both are big enough sites to offer specific targeting within their audience), etc.
(Edit to add that of the two campaigns I compared, both produced a negative ROI from my point of view, i.e. our CPA was far too high - just one was a lot worse than the other. I just pulled up the first spreadsheet I saw from the most recent obvious example I could think of for CTR != ROI.)
If CTR is the only metric you are able to look at, then you can do your best to make judgements on it. But the reason the adtracking solutions that annoys so many people exist is because without proper tracking, it's really hard to actually judge the value of any advertising you buy.
All that said, CTR does still have a big place in the industry, because generally speaking websites (or ad networks) are able to tell potential customers their typical CTRs, but aren't able to give data about further down the funnel (conversions, etc.) because this is data the ad buyers control, not the ad sellers. But it's really important not to think it tells the full story.
I agree that just looking at clicks or CTR can be misleading. Because the people clicking in my case usually can't buy something directly after clicking it can be even more murky. But this particular industry has primarily been advertising in magazines, so in some ways it is actually a better situation than before.
Also, by ~5% CTR in your example are you meaning that for every 100 impressions that campaign was getting 5 clicks?
Yep, CTR is clicks divided by impressions. Although thinking back I believe they were site skins with billboards, not just banners. Even still, for that ad format on that site I would have expected 1-2%. (And yeah, skins do tend to get more accidental clicks than a small banner ad, but in many cases can also provide better results regardless of accidental clicks.)
Depending on the ad format/placement and many other variables, it's possible to have a high-ROI advert with a 0.1% CTR, or a low-ROI advert with 5% CTR (or the exact opposite).
To actually judge whether users wanted to see the advert the best metric would be conversions, but that's likely data the site owner wouldn't have (and even with that data, you'd need something to judge it against - for all the talk that most adverts are ignored, the fact remains that some adverts through Google/Facebook/whatever ad-network can lead to direct conversions with a positive ROI).
Not saying it wouldn't be interesting to hear what his typical CTRs are, just that nobody should look at the answer and think you can make significant judgements based on them.