ordinarily i'd say "any competition is good competition". but the truth is, at some point of saturation, consumers gain nothing.
i think what's happening now is that secondary incentives have developed in the market. running an exchange is lucrative because listing fees have gotten so high---it's not just about trading fees anymore. by the same token, ICO companies are desperate to have their tokens listed
somewhere, which creates demand for these new exchanges.
so it's no longer just about organic supply and demand for cryptocurrencies. i'm not sure how long this trend can last.
Many exchanges = more competition = better exchanges
I'd prefer many choices rather than just a small number of exchanges handling majority of the cryptocurrency market. I couldn't think of a downside as of now.
as long as the order books are thick. if you spread traders and market makers around too much, that makes for thin markets, more slippage and higher volatility.