Wearing wires can restrict developing milk ducts which supposedly causes issues with production and also some discomfort.
My feeling is though, if the bra is well fitting (ie the wires are sat way back off the breast tissue under the arms, and lie flush on the rib cage in the middle) and the fit is checked regularly, is it that big an issue?
I wear both - I have a couple of good fitting underwires from pre-pregnancy that I'm happy to continue wearing, and then a couple of sports bras and maternity bras for other times.