Is there any reason why Bitcoin cannot co-exist with other similar e-money schemes?

Having learned more, I can point out that Bitcoin can co-exist, but those other e-moneys must have some trait Bitcoin doesn't. It's like evolutionary niches. My current example is an interesting little system I'm GPU mining for right now, Namecoin (homepage). The scarce things that Namecoin is decentralizedly allocating are domain names (.bit), rather than tokens used as money - not direct competition.