Why do Emirates want to operate a mixed fleet?

[SLF3]

Emirates operate a mixed fleet and are buying / proposing to buy more A330/340s and 777s, as well as A380s. Educate an SLF: without getting into a debate about the passenger / pilot appeal of the two makes, why do Emirates want the 777? Isn't the 330/340 combination a direct competitor to the 777, to the extent that most operators pick one or the other (e.g. Lufthansa, BA).

Please don't let the responses degenerate into a debate on the relative merits of the two manufacturors control system design philosophies!

[Denti]

I don't know about the other airlines, but afaik has lufthansa a safety philosophy that they don't want to fly over oceans with "only" two engines. Thats the reason they use A340s and B744, but not the B777 and A330.