If I'm reading this correctly, this is saying that to live in a world of ideal forms would be the less limiting option here. However, I usually consider ideal forms to be quite limiting as I generally understand idealism to be so closely aligned with essentialism. For example, when students are introduced to an "ideal argument," their notions of argument become more and more restricted. While this can be good (because it provides models for those beginning), it limits the possibilities of all of the divergent notions of argument.
In the next paragraph, when Lanham says "when rhetoric empowers literature, it is unredeemable. That is what rhetorical literature, I am tempted to say Western literature, is all about," I jump back up here, and then back down there, and then back up here. I wonder if Lanham is suggesting something here about the essentialist notions of humanity... In other words, as long as advocacy for rhetoric carries the baggage of essentialism (like literature as always being boiled down to saying something about the human condition), it will never get beyond the weak defense?