Well, from what I understand about diesel engines, they do not use spark plugs, they use the pressure of the engine inside the cylinder then the piston comes up to create heat and explode the mixture. But I dunno enough to comment much more about that basic function.
Since the SmartPlugs work on the same principle as diesel (no spark) I thought they might add some efficiency. At the least, they should create less drag on the engine since they don't draw electricity when warmed up. If you follow all the links on their web site they have a page where they mention that their plugs can ignite a mixture of 50% alcohol and 50% water (there's your water injection system). Maybe the big advantage would be the ability to burn junk fuel.