Dont why its only fitted to the PD 150
Maybe VW thought with all the extra power the gearbox needs some cooling, maybe thats why your flywheels gone so quick

Julia I wouldn't worry about it the annis dont have it so just remove it when the valance is fitted 
because the TDI 150 is the only MK4 with a FMIC as standard, and all the other MK4s have their air feed in the bumper vent where the external temp sensor is.
looking at ETKA the side cover is a different part number for the PD Anni, i wouldn't worry too much, and you can fix it with some hose
pics below
note parts numbers end E and G (I can confirm all the parts for the PD 150 do end G as I've replaced all of mine)

note no mention of GTI25

be interesting if someone with a PD anni could tell us what it actually looks like