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. But yeah, hopefully we get more decentralized exchanges in the future.
More exchange they more competitive and people more excahnge they can decide to choose to stored their coins this ideas are getting ways to stop price manipulation contorl by the some reach and bigtime companies.