Obviously we can fill up the thread with great amateurs who were awful pros, and awful/non-amateurs who were great pros, but I don't think anyone would argue that a wealth of amateur experience is a HUGE asset to any pro career.

Like mentioned above, it's the expectations set on them that makes their occasional failure stand out.

The reasons for the failures are often personal IMO, and not related to amateur experience not helping in the pros. A lot of times these guys get too much too soon and lose their hunger, or get thrown to the wolves too quickly.