We put our UM2 on a rubbermaid style cart so that it would be easy to move around if need be and there is storage space below for filament and stuff. It turns out that the surface of the cart was not completely flat. I could hear (and then see) the UM2 rocking back and forth as the print head moved from one side to the other. So I took a big slab (2" thick) of high density closed cell foam we had lying around and put it up on that. Then I noticed that the standard "feet" of the UM2 were "sinking" into the foam. So I wanted some feet to spread the load out.
Also I have heard (read) about the electronics of the UM2 overheating. So the feet are also there to improve air flow under the base. I don't know if the overheating thing is common, rare, or just an old wives tail, but I figure it probably can't hurt.
Lastly, I used plain jane PLA, well technically it is Form Futura Easyfil, so I don't expect the feet to help or hinder vibration by themselves. The 2" thick slab of closed cell foam should probably dampen some vibration... but I am no vibration expert.