We don't need a "leading exchange". We need exchanges where all customers can get money and Bitcoins in and out with no delays and no nonsense. All exchanges where assets flow easily in and out will converge on price, because any difference will cause arbitrage transactions to pull them together.
Very good point, in most cases it is very easy to get
BTC and $ in, but getting it out easy is a different story.
Any exchange where that is the case is suspect. That is a
big red flag for any broker, investment, or exchange. It's one of the classic indicators of a scam.
Remember all those bogus excuses from Mt. Gox.