I never really "got" the supplemental draft. I mean I understand it...I just never knew why teams would waste a draft pick on a guy who is either too stupid to stay in school, or was thrown off his team for some reason.
I remember the Broncos spend a 1st round pick on Bobby Humphrey in the supplemental draft causing the team to lose their 1st pick in the real draft the following year. MAN! what a headache that guy turned out to be! It was great he was rookie of the year and all...but he was a pain-in-butt hold out that really put the screws to the Broncos.
Denver traded him to Miami just in time for his cocaine arrest. I think his career ended because he got shot jumping out of a window during a drug deal. I'm not positive...but I think he was the Broncos last attempt to use this weird draft.
I believe the Eagles got Cris Carter in the fourth round of the supplemental. He was trouble then, but he straightened out once he joined the Vikes...and you know the rest.
The only things worse than players that are drafted in the Supplemental draft are players that have to sit out an entire year. For examples, see Mike Williams, Adrian McPherson, Gary Russell, Maurice Clarett, etc.
The supplemental draft may be a "waste," but its less of a waste than forcing players to be away from the sport for an entire year, then using a draft pick on them.
It's generally worth the risk for the teams. I say that because I just don't recall many instances in which a team used its first round pick. They often get players who would be early picks in the regular draft at the cost of a fourth or fifth round pick. Any GM that actually uses the first round pick for the supplemental draft should be fired immediately.