I guess I've seen both sides of this argument in action, though I tend to lean towards one rather than the other.
What I mean is, there is a player in my current group that just made a cool smuggler character concept without knowing much about their motivations or how they'd act. Just started with basically nothing for a backstory, had their name, their character sheet and such, but that was it, so he really only had a purely mechanical character that he wanted to do "cool Han Solo things" with. Bear in mind this player has slight character ADHD, where he loves building and playing with mechanics and so the character roleplay aspects sometimes fall behind.
The last session, events happened where he got to flush out how his character would act in a tense situation(think Reservoir Dogs where half the party nearly killed each other after a run), and it was hilarious and completely believable. No one actually died, but now there's some beautiful intercharacter tension that promises to be fun, and he's having the time of his life with it. He's about the last guy I expected to develop realistic motivations and reactions for his character, and he essentially did no background work to accomplish it.
Now, personally I like to flush out my background a lot. I like to know why my guy acts the way he does, why he turns left instead of right, whether he'll help the wounded merc on the street or finish him off and take his stuff. But I tend towards the other end, where I'll write pretty damn detailed descriptions and information about otherwise insignificant stuff. But I also really enjoy roleplaying an interesting character more than some do, and that's also fine.
My basic rule to go by is very similar to ones posted above, where at the VERY least a player should have a basic concept down for how they act in a situation that is something above the pure mechanical. If they don't, and I've seen this before too, then their motivations become entirely based on metagaming how much loot they can strip off the corpses of their foes once they finish showering the street with bullets. Their only reason for existing becomes to min-max their stats and become the "best" Street sam, decker, runner, etc. Gets really, really boring after a while, and in my experience most of those players also are complete insufferable dicks who like to elect themselves defacto El Presidente of the group.
Anyways, that's my rant. Basic behaviour and motivations for me are a must, but anything other than that is completely optional.
