Hey
I want to say that we do have restrictions on the zeros of a system and they may complicate things a lot.
Usually in cases where stability is of concern, zeros in the right half plane are extremely problematic since they decrease the phase by 90 degrees, acting like some of the poles.
If I am not mistaken, there is a very nice example in Gray and Meyer's book on the effect of the zero and on how to eliminate it. (In the chapter where frequency response is discussed)
But zeros in the left half plane are quite favourable, but since they usually occur at frequencies above that of the poles (Sometimes even above the unify gain frequency), they get ignored.
Cheers