you can't perform a planar on a curve that's not flat... cause all planar does is create a plane... a two-dimensional surface. if you attempt a planar on a three-dimensional curve it'll fail (or actually maya will perform it, but you can't see the result unless you flatten the curve).

as far as snapping a poly to a curve, have you tried holding c (snap to curve)? you might be able to do it with single vertices...