That's the thing, while Gerrard is excellent on the right, more often than not he's not really going to try and take on the full back opposite him (this is not a criticism, its just what I've observed). With someone like SWP/Pennant, they'll occupy the opposing teams' defence much more, which is what Robben (the git) and Cole were doing to Finnan and (to a greater extent) Warnock.