It's a metric we decided on to try and find a fair middle ground between publisher and advertiser. The publisher's job, and only thing they control, is to get a prospects to come across an ad. Whether they read that ad or flip past it has to do with the advertisers ad, reputation, product etc. One thing we know is that 10 seconds is an eternity online, especially on mobile (count it out, you'll see). Since we only offer full page ads and only one page is visible at a time, we like to say that if a reader is spending 10 seconds on an ad they are either reading it or they fell asleep, it's got to be one of the two.
If the publisher gets someone to click or spend 10 seconds on an ad, they did their job and deserve to be compensated. It the advertisers responsibility to test their ads and measure if what they are spending is generating an acceptable ROI. If it doesn't they should modify their ad to try and improve results or pause their campaign.