For daytime grazing, *I* wouldn't worry about a wire bottom.  Aerial predators are not a concern for the bottom, raccoons/opossums are *mostly* a concern during roosting hours, cats aren't likely to dig in -- so really the only major worry on that list during daytime tractor use would be dogs.  That being said, it really comes down to how much risk you are comfortable assuming - if you are a "Ft.Knox" sort that find the idea of even one lost bird to be too much it may be worth it to you to do the base.  One concern with wire bottoms is that chickens are scratchers - a wire bottom may protect them from the potential of a dog digging in, but it also raises the potential of damage to their feet as they try to scratch around in the areas you put them to graze.  How often do you plan to move the tractor?