Since BitShares allows everyone to establish their coin or business on the same blockchain, they can benefit from synergies. Every customer of a competing business or coin that is part of BitShares already has an account capable of dealing with your business or holding your coins. Besides a shared user base, we also have shared markets. You can think of shared markets like a market that trades BitStampUSD versus KrakenUSD, both being IOUs for fiat USD at the corresponding platform. Suddenly, every user of one exchange can be a potential user for a competing business, not just for the exchange business.