Character bio is more for creating hooks and such. As a GM, I'm really just need to know what sort of history your character has. Do they have a criminal backround? Do they own property? How wealthy are they and how did they get that wealth? Background feels like a necessary evil. You could cut it out of your CS, but then how many extra areas are you going to create on your CS to get all the information you need? I also feel like depending on the setting it's a good idea to have some features of a character visible to everyone. I can't speak for everyone, but I like to make unique characters, and I will try to pick backgrounds that set them apart from other characters. So seeing what other people are running with is useful.

But I do withhold things on the CS that I think would be interesting if they came up later, and I never make NPC's sheets too deep. 

Something I did experiment with in the past (and with great success) was to have a secret questionnaire sheet for everyone's characters. So while the bulk of the character sheet was visible, the good stuff was hidden. You'd know how a politician came into power, but not about his scandals. And I think that's a good approach to have with NPCs. Giving some information makes the characters easier to remember, especially if you have a lot of them. However, their secrets should be less well known. If you, the GM, spot an opportunity for a connection after looking through secret details, you can work your magic to ensure said characters know about each other. I love getting secret notes from the GM, makes me feel special.