First of all Old Pokey, my disclaimer, we do sell cover crop seed and what we have learned from working with our customers is that everything you mentioned is important in determining your covercrop choices. What is MOST important varies. Just like anything else in life everyone has a different opinion on what is most important on any single subject. Cover crops and especially cover crop mixes is very complex and add into it different enviormental climates and you have a subject that can be debated for years and never come to any kind of agreement, but that is not  a bad thing. There is still alot to learn about cover crops in todays modern agriculture.

Having said that I would say in no particular order that some of the more important reasons would be cash crop rotations/diversity, soil health/biology, disease and pest management, erosion control, moisture control(either using up unwanted moisture or providing a cover mulch to prevent evoporation),nutrient cycling, building organic matter, balancing carbon/nitrogen ratios, covercrop seed availability, and price.

I think the most important thing in learning about covercrops is that you don't have to be right, it is okay to have something that doesn't work, you just don't want to do it on alot of acres. Try experimenting with some plots and see what works in your area for your farm. Talk to other neighbors that have tried covercrops and see what has worked for them.