There is one argument that fyrestarter 
seems to consistently overlook.  It is
a *much* less efficient use of our resources to eat
meat than a vegetable based diet.  Simply put, you
can feed more people using less land and water by 
feeding people veggies instead of cows.  

Many many miles of rainforest are cut down each day
in order to make room for cows.  Of course, this helps
to contribute to global warming (which I suppose fyrstarter
does not think is a problem) in addition to driving
indigenous people out of their homes.

Sooner or later (probably before we reach 15 billion people) 
there is simply not going to be enough room to grow 
cows and feed all of the people.  Of course, if all you
are concerned with is feeding the approx. 250 million 
Americans and not the 6 billion (or so) other folks, I guess 
this is not a problem.  We can just continue to kill off
non-westerners indirectly by using up all of their 
natural resources and we could probably make those
resources last a little longer.

Of course the simple answer to this is to find another
planet to settle.  This might happen, but it is very 
unlikely that we will be bringing cows with us to eat
on these other planets.  Like it or not, at some point,
humans are going to have to evolve beyond eating cows. 

....

With respect the the antiobiotics, the problem is bacterial
in nature and not cancer.  By treating cows with antiobiotics
we are accelerating the bacteria's rate of mutation.  This
means that strains of bacteria that are resistant to 
antibiotics are evolving *much* faster than if we did 
not treat the cows with antibiotics.  We are seeing the 
return of many diseases which were treatable 20 years
ago that we cannot treat any more.  New strains of salmonella
& e-coli are great examples of anti-biotics injected into cows 
accelerating the rate of evolution of previously treatable
bacteria.

For more information on this & cancer (which, in fact, 
can sometimes even be caused by viruses) I suggest you read, 
"The Coming Plague" by Lauri Garret.