You could if you want, but chassis platesshould have some flex, and sometimes, that can be a good thing. Again, if the aluminum chassis plate is involved in a crash, it will be "tweaked" How bad depends on how bad the crash was, but it will be permanent.

If I were going to do aluminum, I'd stick with stuff like bulkheads, shocks and shock towers, C-hubs and steering knuckles, rear hub carriers, prop shafts (for shaft-drives), and motor plate. Some small parts like ballstuds, and nuts can be, too, but the rest? I stick with plastic. Plastic is cheap, easy to replace, and comparatively more durable, because it will bend before it breaks, and won't stay bent like aluminum will.

It also keeps cost down.