I suppose the wire ribbing might be partially visible when the fly is wet, and in the water, glinting through, etc, and the colour won't change as wet thread might. Also, wire would add a touch more weight, and by tying in the wire, the body is thickened (which may or may not be desirable). In the end, though, the weight would be minimal, the extra colour glitters could scare some fish while attracting others, and desirable body thickness depends upon the fly: so, tie 'em as you want!

- Jeff