You don't need it, and possibly don't even want it. You do want a Sto-Fen or similar push-on diffuser, though.
I know Stan knows what he's talking about, but I had some downtime this afternoon and my normal models are off watching the new Muppet movie with friends, so I decided to experiment a bit.
Westcott Rapid Box 26" Octa with Cheetah Light V850 (radio trigger hotshoe flash). I dialed the flash power back to ¹⁄₁₂₈th, zoomed it to its widest setting, and selected a relatively narrow aperture and low ISO.
I took three test shots, one with the flash unmodified, one with the wide angle diffuser pulled down, and one with a Sto-Fen push-on diffuser. (See my previous test with the Rapid Box 10"×24" Strip .) Then, I repeated this with the defector plate in place.
As before, the bare flash clearly doesn't produce enough spread. Unlike the previous test with the smaller rectangular box, there is a clear advantage to using the push-on diffuser over the wide-angle panel. The extra internal diffusion is particularly interesting in the last test where the plate ends up with an... umbra and penumbra, I guess.
The plate casts a considerable shadow. It might be useful in some cases, but for most even light, use without the plate but make sure to use a push-on diffuser.
Here's a direct shot without the front diffusion material, in "beauty dish" configuration. (Push-on diffuser in use; it makes this better too.) Kind of an interestingly-distint spider pattern:
And, that pattern is actually fairly clear in catchlights, which some people might not find a nice as a pure, clean circle. If that's a concern, using the plate with the front diffusion material might be preferred (although then, of course, you're not really getting the harder beauty-dish look).