I'm a terrible sight reader, and I work at it everyday to become better. I sight-read hymns, children's music, easier Bach pieces. I try to do everything I can to become better at it. Then again, some people are just naturally good sight-readers, and others are not.
As much as I hate sight-reading, I do it because it is necessary. Sure, being a good sight-reader is not absolutely essential, but it helps so much in the learning of repertoire. Being able to read accurately the first time through a piece, even at a slow speed is a big benefit. Being a good sight-reader helps you learn music faster allowing you to build a bigger repertoire faster.
What it boils down to, as far as solo piano goes, being a good sight reader saves time learning the notes, so you can spend more time learning the music. I'm already very good at memorizing, and I make up for a lot of time because I can memorize things very quickly. But being a piano student in a university studying to become a professional means I can't just work on solo repertoire all the time. I am also required to accompany and do chamber music. Being a good sight-reader is such a valuable asset in those respects.
So to the pianist who plays just for themselves, being a good sight-reader may not matter all that much. But to someone studying to become a professional, or who already is a professional, sight-reading is a highly important asset.