Unless there's a runner/runners on base. If someone comes to the plate with runners on base, I'd like to know the percentage of time he has hit safely (along with the SLG to know how likely they are to hit a XBH)
Average is important because it tells you how often the OBP is a result of a hit. I think SLG would be better understood if it was called something like bases per plate appearance or something that sheds a little more light on what it is.
Let's look at the probability of Branyan's at bat vs. a lefty based on this year's stats: Probability he: Strikes Out: 60% Takes a Walk: 5% Gets a Hit: 5% Hits a homerun: 0%