Reason mostly liquid metals is used under the lid is heat. A) the metal will conduct the heat far better than any paste will, a good thing for a die trying to pass the heat to the heatsink. B) metals don't dry out, powder or deteriorate the way paste does, so really it's a 1 shot deal, not something that should ever need redoing the way some ppl feel the need to repaste if their cpu goes up 2°C.