While i agree that you are saying your machine works well as it is in mdf and does the job that you want it to do, I'm sure there is more reason behind everyone's arguments to use metal than just do it as that's the only way, fact is mdf bends a lot. Now the only downside I can see to this flex is that your pieces will run out by a certain amount, this is quite obviously within your tolerances so great for you. I think you may get even more play from your 8 x 4 machine but again if your happy with it then super.

I originally planned to use mdf for my machine but I wouldn't want to make it and it to have play in it, so I've decided on steel box section as I see this as the next step up from mdf/ply