My 2510p keyboard was on a metal plate. It flexed a bit but once screwed in it was very very sturdy.
It depends on the laptop: Some manufacturers use the keyboard as part of the top case, and those are quite stiff. Once you remove it, you can access the ram, etc. I don't like that design.
I but it also had 2-4 screw mounts on the bottom, too. It's a HP, right? they like to do that sort of thing.
On the other hand, you can have all the components accessible through the bottom, and have the keyboard rest in a niche on the top case. In the case, the keyboard doesn't contribute to the overall structural integrity of the computer, and it'll be flimsy.
I guess there's a third design: have the keyboard be inseparable from the top case, but this is really bad if you spill on it (and all the components attach to it) 'cause then it's super expensive to repair. The apple unibodies are like this.