For most applications they are not needed. I did some extensive testing with spacers a few years ago, street and at the track.
I built and tested the following:
4 hole 1", fully open 1", fully divided 1", semi-open 1"
All were street and track tested. The 4 hole provided slightly smoother off idle throtte response, but ran slowest at the track. The semi-open was about the same. The fully open was the worst for street driving.
The sem-open ran the most MPH at the track, but was .02 seconds slower than no spacer at all.....Cliff