[asshat]

So by the fiberglass assumption.....

if you were to be using concrete for say a parking lot... then it would be a good idea.. but it would not be a good idea for a column

Actually, I think that fiberglass is used only when steel is either not economical or is not feasible. If you are making a lightweight concrete slab, say a parking garage deck, they might use fiberglass to reinforce the slab to cut down on the weight. I have never seen fiberglass used, so I tend to believe that the tensile strength that it gives the concrete is not as good as steel.

To answer your question, it would probably be better used in a compressive situation like a slab, rather than a tension situation like a column or beam.