Someone uses several exchanges, and someone needs all 10-20 exchanges.
My opinion is - a lot of exchanges are healthy competition, improving the quality of security and anonymity.
For the increasing numbers of exchanges, they will should be providing good services to their clients/investors to make them stay all the time otherwise they will left and find new one. That is exactly a competition and for those exchanges that have already establish good market history will taking advantage against the other and most probably investors will never have a way to transfer.