To me eCPM is not an intuitive number. Oh.. $10 today. Oh $50 today. Oh $4 today.. For me it is all over the place and doesn't mean much. I know I want them higher, but that's kinda arbitrary. $1 per click (my target) with 1% CTR is 1 cent per unique. (if you only show them a single page with ads to a unique.) Using this gives me very specific traffic goals. If I want to reach my first big goal of $28 a day (minimum wage), I'm going to need about 2800 uniques a day. It's easy enough to compute this other ways, and of course the math is all the same, but saying it like this is much more intuitive to me.
It CAN be a very useful tool when deciding where to spend your TIME. If you had 5 sites and you see low visitors to one site the tempatation is to concentrate on the other sites. However if you "see" a high ecm on a site you dont spend much time promoting then it can be a trigger to give it some more attention. I had one site with an average daily visitors of 5 (compared to another with 2000+). However having spotted a far higher ecm on the low volume site i put some effort (and money) inot it and it is now a big payer.