The only college seniors drafted that go unsigned and unassigned are those that choose not to seek a career in baseball. All other college players drafted have to choose between college and pro's, they lose no elligiblity by being drafted -- there is the famous case of Lance Blankenship who in his junior year was an all-American and finished third at the CWS, turned down a reported $300,000 signing bonus (in the mid-80's before greed really inflated prices, today's equivalent would be $2M at least) and returned to college for his senior year thinking he would win the CWS and be national POY. Instead, his team finished with a losing record and he was barely honorable mention all-conf. He was either undrafted or picked really late and signed for a standard low-paying contract. He eventually made the Bigs but was not fast-tracked to the Majors the way he would have been had he signed after his junior season. High school seniors drafted can either sign or choose college (again, eligibility is not affected). Occasionally high schoolers make a quick jump to the Majors (pitcher Mike Morgan made his Major League debut a week after his high school graduation), but that is the eception not the rule. It is one reason why good college teams are considered better than most Class A teams-- the Class A team is younger.

The problem with college teams is that a lot of them don't play enough games for the players to really develop, and, paradoically, a lot of the better teams used to, and may still, ruin good starting pitchers by overpitching them.  It wasn't all that long ago that top pitchers would pitch some huge percentage of a team's games, and it may still happen.  Ben McDonald, for eample, was a guy who could have been a real major league stud, but his college coach (LSU?  Teas?) way overused him, and he was damaged goods when the Orioles signed him, and was often hurt in the majors.  Clemens actually went through the same thing at Teas, and ironically, it was thought that he wouldn't have a long major league career because of it.  The Red So had to kind of baby him for a few seasons until he got over it.

An interesting fact:  Danny Goodwin, a catcher, was drafted #1 at the end of high school, went to college, and was drafted #1 coming out of college.  After all that, he played 252 major league games with a .236 average.