I linked this just the other day, and it's relevant again. Life in prison for stealing a pair of socks: http://www.rollingstone.com/politics/news/cruel-and-unusual-...

If that's not manifestly unjust, I don't know what is. I have no idea how 3 strikes laws and mandatory minimums are constitutional in the USA, I'm no constitutional scholar (or American), but I'm fairly sure that it violates separation of powers.

Regardless, the above link is a very good reason for judicial discretion, among other reasons.