I am impressed with people who can sightsing.
It's like sight reading on your bass but with your voice. I believe it helps with your musicality, therefore your bass playing. You may never have any intention to sing in public or with your band. Learning to sightsing opens up another musical part of your brain.
Picked up this Musicians Institute book and am enjoying going through it. It starts at the very beginning so even if you can't read a note with your bass this will teach you all you need to know to sightsing (and read music). 158 pages. The book was $7 used on ebay with free shipping