A plastic putty knife will not hold a straight edge. The plastic wears down and due to the nature of plastic the manufacturing of the knife is not as precise unlike a metal one which will hold its edge.
Hey Bob, How's things? I'll pile on and agree with brush and mickey, plus I'll add that I think it's harder to smooth compound with plastic. Personally, I like thin, flexible blades and plastic knives are too thick for my taste. Take Care.