One way to look at it is that they prevent a bank run. The other perspective is that they trick people into thinking the market price is higher than it really is, making unaware people lose even more money as they buy at the high price and the crash happens anyway, just over a longer time.
[1] http://www.malwaretech.com/2013/11/mtgox-nearly-breaks-bitco...
So you risk your (incredibly profitable) business just to skim a little on the spread? Doesn't seem worth it.
A lot of people who just do not understand how thing kind of market works make these allegations— due to things like issuing a market order and then having the trade execute at prices which are dissimilar to the last price but completely explained by the published orderbook.
I think it's very possible for exchanges to do this sort of thing, especially when their technology is so poor that it's difficult to audit how they match orders.
They are _a_ market. While they may historically be the leader, many bitcoin trades and negotiations never touch them any longer.
This no longer belongs to a single site. It's too big now.