Read Ridley ("The Red Queen"), his data are quite compelling. Length of
generation is almost a direct correlation with sexual reproduction, and
the genes dealing with histocompatibility and immune response are those
with the greatest variation in the gene pool.
While it is true that the population of immune cells in your body itself
evolves to match the germs it finds, it is only able to do so because it
can recognize bugs that don't have all the right passwords; but it is not
possible to change the passwords themselves, because they are hard-wired
into your DNA, so eventually the bugs evolve to have the right ones. Your
genes can only change the locks by producing offspring with scrambled DNA.
It's a fair gamble, 50% live long enough to breed, instead of 100% dying.
Just look at the lack of immune vigor in dangerously inbred species like
Cheetahs. Cloning is the ultimate inbreeding. No variation at all, no
chance for new combinations, no new locks on the old gate. Every farmer
knows the same thing about his crops: once he finds a good hybrid, he
can asexually propogate it for a few years, but then it will succumb to
diseases, and he must rehybridize for new vigor.
Cloning for spare parts might help extend our brief lives a bit, but it
doesn't change the basic fact that we are designed by evolution to grow,
breed, and die. To get past that, we need a new design entirely, not
based on cells vulnerable to viral and bacterial attack.
-- Lee Daniel Crocker <lee@piclab.com> <http://www.piclab.com/lcrocker.html>