Life as an unsharpened pencil
The sign works as a metaphor, not just a silly pun, because we're meant to understand an unsharpened pencil as pointless in two senses -- both that it is not physically sharp, and that it is useless. The latter sense is based on the idea that a pencil has one and only one purpose in the world (to write), and it must be sharpened for that purpose. This is like how many Christians think about people -- we were made to serve one specific purpose. God has a plan for what our lives are supposed to be for. And if we aren't "sharpened" (by faith in God and religious practice), then we can't serve that purpose, and our lives are thus meaningless.
But writing is not the only thing you can do with a pencil. You could use it as a chopstick, or to poke into a drain to clear a clog, or carve it into an art project, or use it as a makeshift capo for a guitar. Some of these purposes may not be optimal, but it's fun and interesting to try to think them up. The pencil is hardly pointless if it's being used for something other than writing. And that's how I see people -- there is no one purpose dictated for our lives.
The point of my life is whatever good point I can find for it. To have a religion telling me that there is one and only one thing I should be doing with my life would make my life feel ... well, pointless.