Serving certain targets - be they Zones or passers - can be very useful. Let's not forget, though, the one actually having to make the serve.
There is a point at which service aggression is just right to achieve the highest point scoring (breakpoint) percentage, but it varies.
A fellow blogger took a look at scoring points in streaks in the volleyball and how it relates to the outcome of sets. I have my thoughts on the subject.
Lots of volleyball coaches want their teams to serve tougher, but there's a trade off between winning more rallies and losing more points through higher errors.