Are you talking about the metal pieces that slide into the intake ports in the cylinder head?? If so I know of one car (stock turbo) that after a decarb had them left out by their technician (me, oops). That was years ago and that vehicle didn't get them back until the next time the intake was off a year or so later. Everything I have ever read about them was to increase swirling of the air at idle and/or very low rpm for a more complete burn. To me always sounded like a bunch of emissions save the penguins kinda stuff. So all of that said I do not think there is any reason to leave them in there aside from a place for carbon to collect before it gets to the valves....