The idea behind a throttle body spacer is to mimic a carb spacer. On a carbed engine the spacer gives the air and fuel more time to mix together. Since a fuel injected engine injects the fuel far after the throttle body it literally does nothing for you.... Just creates a longer path for the air to travel. As HemiLonestart said the only reason to use one would be to run nitrous.
I have a 1998 Ram 1500 with the 318. I have a spectre cold air intake and throttle body spacer. I didn't plan on to much of a difference, but i did notice the quicker throttle response. I did have a problem stuffing the intake on after putting the spacer in, but that's nothing a hammer couldn't fix.