I heard about the 100 day trial in a forum post. A friend, highly dissatisfied with a site, found out on the 45th day that the site was a huge pay out. Although it was dissatisfying at first, because of the slow income start, they quickly fell in love with the site. (In case you are wonder, they fell in love with ehow. Which is now demand studios) If the friend would have given up when the long slow start, she would have missed out on a nice pay check, and a new good writing site.
Many sites take time to learn to use, article take time to mature, and therefore giving new sites a 100 trial period is the best way to see if a site is really going to work for you. Each writer is different, therefor not all writing will like the same sites.
When you stumble upon a new site, give them the 100 trial period before leaving. This will give the site a fair shot to see how it really matches up with you.
Remember - Never pay money out upfront; 90% of the time it will be a scam.