I think the better the quality of plastic cabs, the better they will be.
I doubt a cheap plastic cab will sound worse than the equivalent cheap wooden cab, if you see what I mean!
It would make sense that the thicker and/or denser the plastic, the better. Thin plastic is bound to rattle or flex and affect the sound quality.
Not sure about bass bins though. The plastic would have to be so thick and dense to prevent rattles as mentioned, they would most likely be heavier than their wooden counterparts, as someone has already mentioned.
If it sounds good, who cares what it's made of as long as they work for you.
People have scoffed at my mdf/plastic Jamo cabs, but I've had plenty of compliments about how good my sound rig is, especially from people who know nothing about makes and specs, so there's yer proof!