I'll let the ShopBot guys chime in on why they picked what they picked, but I can say for a baseplate that is near the tool, I like them to be out of something millable, that way if the tool ever hits it, it doesn't matter.
At one point I had a base that had the exact opening of the cutting area of the Handibot on it. The problem with that is that if I wanted to cut something that was 6x8" the tool would have to travel farther than that to cut out that shape. It can do that, but with my 6x8" opening, I would have hit the sides of my jig. Because my jig was wood, this was fine, if it was metal, that would be a problem..
Does that make sense?