As mentioned, Coinbase is not an exchange and honestly leaves much to be desired (limited instant buys, for example). I think the better reason is that allowin

As mentioned, Coinbase is not an exchange and honestly leaves much to be desired (limited instant buys, for example). I think the better reason is that allowing any single Bitcoin trading company to get too big is unhealthy (see MtGox). Having competing companies is very much in the interest of the btc community.