I was wondering this as well when I bought mine. Paul Marcel showed on one of his blog entries using the widener on his of 1400. He ran the router off the rail on the side opposite the anti-splinter strip. Since I thought that was how I would prefer to run it, i bought the widener base.
I haven't had a chance to use it, but the last time I was routing I was switching between the guide rails and edge guide for rabbets. The widener won't work with that, the top of the edge guide won't clear the bottom of the base. And, its pretty cumbersome to switch between the normal base and widener base. I ended up using the foot and it worked really well. Because the bottom of the regular base and the guide adaptors are coplanar, the router is nearly fully supported on the guide rail. The stop feels like its there for more peace of mind. After using the foot, I really think the widener base will stay in its box unless I have a lot of guided bearing bit routing and dadoes.
I wish Festool would just make a snap-in 5mm spacer plate. It would be stupid simple to make and I'm sure (based on queries of this nature) that they would sell quite a lot.