One of NASA's upcoming projects is a mission to Europa. With current technology the travel time to Europa would be about 7 years. With the SLS it would be cut to about 3-4 years depending on the launch window. SLS would not the the end game, as you'd have to keep improving, but it would allow us to more easily and quickly explore the entire solar system and beyond. We need the SLS.
As of now, it's about smart spending, but I do want to throw this out there: if NASA's budget today was equivalent to what it was in the days of the Apollo missions, it would be approx. $100 billion. Instead of nitpicking what we fund and don't, let's throw NASA (more or less) that level of funding so that we can expand and explore the solar system and beyond.