Google, you tell us that CPM ads only show when we'd make more money. Well we aren't so sure it's good for our sites. If your contention is right, it will be clear very quickly. Just let us turn it off and on. We'll flip the switch and test and if we see it'll make more, we'll leave it on, if not we'll turn it off. Good for you, good for us.
Doubt that will ever happen - too much hassle for the people paying the bills -the Adwords users. If you don't want CPM ads on your site block the url when you notice them appearing.
CPM ads can be frustrating. Seeing a click and no $$$, ya that's frustrating. But, seeing $$$ when there's not clicks is nice. My site is targeted about 1/2 of the time. I find, I make more money when it's targeted. That is, CPM tends to pay better. Mileage may vary.
Seems silly to opt out of CPM ads to me, but well... What I would like to see is you being able to see what CPM ads are setup to run on your site (but I don't imagine that will happen either)... Although if you could, then you could block them by URL pretty easily, so might be a compromise. For me, I'm just curious how many CPM ads are running total.
I guess what I was saying in a fancy way that if we actually knew what was going on with them, we'd feel more comfortable. We are totally blind as to what's going on on our own sites.