AIUI it positively stops the collar from moving to the 'release' position. There was someone on the CTC forums complaining that without them and using the MKS SPD-a-likes they could sometimes nudge the release and pull out the pedal instead of the shoe when trying to unclip...
My Brommy currently sports Ti Workshop pedals with similar mounts.
No Plasticky yellow is deployed.
No pedal has parted company with the crank in around 3 years of use.
I have some MKS removable pedals, still in their box.
Exactly what function does the yellow plastic bit perform?