New members being able to 'declare ' their own rating seems odd to me.Why any need to change the old system which worked perfectly well.
I suppose it doesn't make much difference in blizt or bullet since the ratings will find their "true value" quickly but in correspondece chess where games take months it's different. It used to be the case that when an experienced otb player joins he will for a long time face mostly much weaker opponents which can be quite frustrating for all parties involved.
I have found at more than a dozen correspondence sites that when I first join, I have to win all or most of my games for several months before I find my level at that site and get good quality competition.
If I could self-declare as "above average", it would shorten this orientation period. In my experience, one other site asked me to estimate my level when I joined: ChessWorld.net. I am still playing there twelve years later.
It may also be worth mentioning that the possibility to choose one's initial rating (within certain bounds) is not unique to Chess.com. I think that for example ICC also has that. It's certainly common practice among different Go servers where even fast games typically last longer than in Chess.
Also, in otb tournaments it's possible for organisers to use an estimate for the initial rating of a new player if there's some reason to believe she is significantly stronger than your average beginner.