Well, in my case, it is necessary for the overall story.
If he remembered then he would most likely be a bad guy, and City of Villains is not out yet so I can not do that 
. Plus it makes for a very open story line, and since we are working with literally 1000's of variables the more open the story the better.
I do thing the author needs to know the past even if the character or the rest of the public does not. Never let your audience know more about your story than you
Most of the time it is not lack of a good story, it is just using one of the available plot devices to your advantage.
For the record my character doesn't have amnesia, and I did write up a detailed origin.
Don't worry about what another player has written to describe his character's background. It's their game and it's their character. To be honest it's refreshing to see that someone even bothered to write a background at all.
There are lots of people who play this game to role play, but others do not. Sorry but you can't make them. People are going to do what they are going to do. I am a roleplayer, but I don't expect everyone to role play along with me. If they do, swell if they don't thats fine too.
The only thing that bugs me in this game is when I see a really interesting costume... a REALLY great name... and then, no origin.
I actually choose who to talk to usually based on that fact alone.
There are also people, such as Ivory, who's descriptions are very brief. I like to think of that featuer of the game as my character looking at someone and saying to himself "Oh hey, I've heard of that guy, he was mauled by radioactive aligator mutants from outerspace and now seeks to avenge his sad fate by hunting down the political monsters who's budget spending allowed such abominations to be stored at his local japanese bath house located 1200 years in the past. The only way he can get to them, however, is to make a name for himself so great that it cannot be ignored.
*heavy breathing, catching breath*
So as a result, Ivory's description is simply (and roughly): A brilliant engineer and expert marksman, Ivory has tried his hand at many occupations over the years. He hopes to curb his boredom with heroism and seeks to clean up more then just the streets of Paragon City.
Very simple, very concise, but it's also all that i want the average hero to know about me. I just figure people with more information in their profile want players to know that about them.
The point is... for some players the lack of information (in some cases amnesia) is simply a way to establish that there is something more to the hero that the average public doesn't know about and perhaps by getting to know that person you'll be able to learn that past.
God damn I'm long winded... I should write a book but... who would buy that?
I'd buy yours if you'd buy mine!
Yes excellent point. I don't discriminate on the size or detail of the origin, merely if it's THERE or not...
And it's not ALWAYS proven true. I do know some quite excellent and fun RPers who have not gotten around to filling in that spot yet.
There is always an exception to the rule. It's really only a "first impression" anyway. 
While the amnesia backstory might not be one of the more original ones nowadays. Some people manage to pull it off excellently by either giving it good reason or adding a twist to it.
As long as it's pulled off nicely, I have no problem with it.. ^^ Not that I swear about it otherwise either.. =P
Well I have about...6 characters. My main characters has an amazingly in depth history. In fact, about three other charactes were created as spinoffs of her story line and I've created their stories in depth also.
But, then I have Espial, a villan acting as hero (because no City of Villans), she is very enigmantic. Her info is roughly: Of all the enigmatic individuals associated with Crey Industries, Espial is perhaps the puzzling and the most deadly...
What I know of her history is very basic. I know of her from when she enters the lives of my other characters, but I have yet to come up with anything before that. Why? I actually think it adds to her, the fact that even I don't know her story.
It's all a matter of creativity. While an unknown past can work well for some, I think the major bone of contention here is the MYRIAD of bios that ALL say "His/Her Dark and Mysterious past is not known even to the few who know him!"
You can usually tell when someone is being creative by including mystery and the unknown into their character... or when they didn't really care to do anything and decided to just say "Unknown past..."
But that's STILL only first impression, if they turn out to totally surprise you in conversation, then excellent.
Just saying that from experience, when I talk to someone with no bio, I get "Want hlp me bust thru a few lvls?? I got mad skillz! My fingers hurt from all the fast key pressing dud3!"
That just makes Sly go, "When they find you, tell the boys in the white coats I didn't bring you in out of pity..." Silly heroes... why must these escaped mental patients always find me?
Of course, I'll usually ALSO say if they seem offended "OOC: no thanks! I'm busy right now, but thanks all the same!"
I've read too many heros' backstories in whcih there heroes has a mysterious past, unknown even to themselves. It's getting on my nerves. I wish some people would actually write themselves a good, coherent story. I mean, who remembers that they have superpowers and to fight crime when they can scarcely remember what they had for breakfast?