As regards the "plastic" scheme, I'd guess it's possibly not that widely used (any more) and it has a number of quirks that are tied into its history...

It was specifically created (by Mike.S. I guess, IIRC) to look "nice" in OSX back in the 10.small_number Aqua theme days, and it enforces a number of features that were specifically intended to make it blend into the Aqua look. (And to prevent users from doing things that would not fit in...)

Like the unusual colour averaging behaviour, and the strip tiled backgrounds, and so forth...

It can also be "hard" to render in some cases, though why is not clear (on Win32, with GDI+ enabled, some machines, but by no means all, make a real chore of redrawing the "plastic" theme and can be quite sluggish. No one knows quite why.)

So I'd suggest not to use it, Just In Case! 

The "gtk+" theme is somewhat similar to look at, but seems to be cheaper to draw. Though it was styled to play nice with the Gnome Humanist style, and as a result also has some quirks pertaining to colour settings...