Assumptions...

There are several obvious assumptions that have to be made
for a space combat game to work, these have already been
made in the Tuffleyverse (did I spell the correctly?).

1.  Space travel is relatively inexpensive.
2.  Colonies have been put in place.
3.  Shipping goods is a winning proposition.

Jump up and down about how realistic or unrealistic assumption
X is, it doesn't matter.  If we don't have these 3 things
(and probably others I didn't think of) we don't have a game.

If you want to play a starship combat game in a universe that
can't support starships because you don't have one of the
three feel free.  Leave me out.  If you want to play DSII 
or SG strictly on earth that works and is fine.  Most of
us like to play on alien planets and environments (my biggest
complaint is no null-g rules) so we have to assume that there
is a reason to fight.  You don't fight over things you don't
want.  Whatever your personal opinion is on there being
colonies, it doesn't matter in this context.

Roger