It is because, from a purely secular point of view, it ensures
the survival of the religion at the very least. Also, having more
members, means being more accepted (I'm leaving politics out of
this).
But what is wrong with evangelising with your ACTIONS? Strange
concept, but rather refreshing isn't it? We could live the Gospel
and show love and care to those around us, irregardless of their
religious preferences. One day, someone will ask you, "How do you
keep your cool when everything is going against you?" or "How come
you're always so nice to others?"
Then you reply them, "Because I'm
christian/muslim/buddist/taoist/etc" or something to that effect.
I'm sure they would want to know more.
BTW, shade, I'm not directing this post at you. Its directed at the
christians. 