It depends.
A "realistic" set is needed if you really *need* the underlying mechanics/lore/whatever.
If you don't, then you don't need it.
Two examples : In the TDE role playing system, everything MUST be consistent to its rules system - BUT there are still a few "holes" which cannot be explained very good and upon which fans debate.
Second example is LOTR : The underlying worl (no mechanics there at all !!!) were developed throughout DECADES ! The underlying worl was originally the FIRST focus, and like a Tell
http://en.wikipedia.org/wiki/Tell , the LOTR novels are just nothing but the top layer upon everything (else) ... There is no "realistic" world needed, becaue everything was built from the ground to the top.
The point where a game world needs to be realistic is when an imagined/invented content of a world
is needed to be explained. Either explicitely or implicitely.
I could invent a world from scratch . it wouldn't need to be realistic.
Realiric it must be only when it meets something else - hence the
context given is important !
Sorry for my grammar, my mind is kind of exhausted today.
