Taking the theory used in pushbikes,alloy is lighter and stiffer(so great for cross country)less weight to push around and less flex means more of your pedalling energy actually drives the bike not flexes the frame.more hardcore applications like jumping are better suited to a chromoly frame because chromoly is naturally a spring steel,meaning it will flex more with a big hit, reducing forces at critical weld points and such.Alloy generally will crack and snap,whereas chromo will flex and bend first.
So using pushies as an example most(not all) bikes built specifically for jumping are chromoly framed.
downhill bikes(aliminium framed usually) are usually jumped pretty big but the exception is that they also have huge travel QUALITY suspension.
there is of course views on both sides of the argument, but personally i would rather land a 30 footer and have my frame bend and be stuffed than have an alloy frame snap and lead to all sorts of carnage(you can ride out of a bent frame ,not so easy when your bike is in 2 pieces).
As for the klx plastics i think they look **** hot,alot easier to hang off back of bike without pointless sidepods sticking out,and actually looks like a real dirt bike