From what I've read on the subject there are a few reasons to use it.
1st. Despite some vehicles being less efficient on it, mixing at even 10% causes your emissions to be much cleaner.
2nd. Ethanol is renewable (we can grow more), oil isn't (we use it up and it's gone).
3rd. The more we can reduce our dependancy on oil from other countries the better. I'd love to see the day that we can walk away from the middle east and Venezuela and let them self destruct.
To see the emissions difference visit http://www.fueleconomy.gov
and pull up an E85 vehicle. Then look at the carbon footprint between E85 and gas (I used the 2008 Tahoe).
I've seen a number of areas where the blended fuel isn't any cheaper (Appleton was the same way until you got to E20 or went to a Renew station which is in a partnership with a refinery (and being sued in Green Bay for charging less than the minimum markup).
My real question is why haven't we gotten rid of the stupid minimum markup law. Let the stations COMPETE! Heck, for the first time ever I'm seeing some real differences in prices at stations but I had to move to Madison.