The only reason I'm not using conventional dino oil is because I'm buying synthetic blend oil for less than dino oil. I am a little more comfortable going 5,000 to 6,000 miles on the blend than I am with the straight dino, but it's not the reason I do so.
As others have said, I don't like the idea of running any oil for extended periods of time, like 10,000 miles. True enough, a good synthetic oil will last 10,000, but I don't want the ever accumulating contaminants running through my engine for 10,000 miles, too.
Now, if I bought vehicles when they were brand new and knew the history and had ran synthetic from day one I'd feel better about 10,000 mile synthetic OCIs, but I don't buy new off the lot. I prefer for someone else to take that big financial hit.
Well designed, well maintained engines have been going well over 200,000 miles for a very long time on conventional dino oils and until I have an engine that has to have synthetic, or if it makes sense from a financial point of view, I won't be using it.