Assuming that is the same link that Duckman posted from 2005, Doesn't it more or less say that the ducks we are killing with Mojos are ADULT and not juvies, as opposed to the opposite in northern states?

I think that studies of ducks taken over mechanical or regular decoys would pretty much bear out the same as well. Young dumb ducks dying first. Hardly earth shattering information.

Why, other than the aggravating noisy nuisance that they are, do we want them outlawed again?

There are plenty of identifiable hunting tactics that cause thousands of times more ducks to be killed than mojos each year. Those tactics are being refined and used in areas now that even 10 years ago were unheard of and they are negatively affecting the migration to Southern states. What was once an Atlantic Flyway problem is now turning out to be a north/South issue as people realize that not only is it not so expensive to overwinter large numbers of migratory waterfowl, the US Government will pay for a large percentage of it even on private property. Spinners are not even a minor factor in what is at play...