I agree, at least on my end, I want the timer to start when the site loads so my ad is seen and I get the most from my credits. I do see a problem with many counters though that seem to "stick" while a video or other animation is playing even after the site is completely loaded. I now it is the video or animation since if I click the video to stop it playing, the counter starts again.
I have even seen some sites had a message that basically said, "You don't have X enabled, sorry for your luck". Which tells me the site owner cares more for some whiz bang bell and whistle on their site than for people to actually see the site. I don't know why people think this way and it doesn't make sense to have a site that won't work for everyone. Why exclude people who could be customers just to have some feature on your site?
I guess it goes back to being a conscientious advertiser and not putting slow loading, high resource, auto play video, or other such type of sites on a TE or safelist.