The RP I'm currently GMing started out that way. RPers would write 3-4 posts covering everything we needed to know about their characters in what we eventually dubbed as 'the flashback phase'. All these posts took place before the major event that the RP is centered upon. Everyone was required to write up sheets afterward (which wasn't originally intended), but I had never taken that approach before and I thought it was fun. I would do it again, depending on the RP. I don't actually value sheets all that much, it's just easier to refer back to one when you need to find something out about another character. Going through posts is much more tedious.