In the US prisons are big business, with lobbyists working hard to get more people in prison so there will be a need for more prisons. (mandatory minimums and three strikes are just two such measures that from the outside looks like they are designed specifically to create a need for new prisons)
As such it's really hard to compare the US prison system with those of other countries and it's also difficult to discuss the US system from a law enforcement perspective since law enforcement was only one of the deciding factors in its design.
And to follow up on Neverfly's post, this is a likely button-presser subject, reread your posts twice before posting them.
Reductionist and proud of it.
Being ignorant is not so much a shame, as being unwilling to learn. Benjamin Franklin
Chase after the truth like all hell and you'll free yourself, even though you never touch its coat tails. Clarence Darrow
A person who won't read has no advantage over one who can't read. Mark Twain