I've found no real correlation between a roleplayer's ability to fill out a bio section and their writing ability. Nor do I see any link between bio length and RP success rates. How effective they are needs to be evaluated on a case by case basis, but it ultimately depends on how the GM chooses to run things.

I don't mind filling out bios, but it's typically not very clear how much or what type of information the GM is looking for. I like to surprise other players, so I usually keep the bios pretty light. But it also depends on how the OOC is written up. Obviously I'm not going to write a 2K word backstory if you're going to describe the world in two paragraphs.