Plastic intakes(upper plenums) have been used by many starting as early as '96(GM).Plus it's cheaper,by alot,and easier to make then a alloy plenum.
I agree here that plastic is much cheaper to produce then alloy, and I read too that inner channels are much smoother hence giving you better flow. But to me it's just cheap, but if you shoudl need to replace it, they will still charge you and arm and leg for it lol.
On another note, again just kind of dreaming, I think having an addtional option of the 3.0 CRD and a 4.7 Liter V8 with cylinder deactivation would be great optional engines for the Liberty matched with a 5-6 speed auto or man trans and some factory lockers.
Moreover, I am happy with what I got, and only have 38 more payments left to go :]