The requirements for these lights are as follows,

All motorcycles must have a functioning red light mounted at the rear of the machine to be used in rain or low visibility conditions as instructed by Race Control. The team must ensure that the light is switched on whenever a rain tyre is fitted on the motorcycle and/or when any practice or race is declared “wet” by Race Control.

Lights must comply with the following:

a) lighting direction must be parallel to the machine centre line (motorcycle running direction), and clearly visible from the rear at least 15 degrees to both left and right sides of the machine centre line.  

b) mounted on the seat/rear bodywork approximately on the machine centre line, in a position approved by the Chief Technical Officer. In case of dispute over the mounting position or visibility, the decision of the Chief Technical Officer will be final.  

c) power output/luminosity equivalent to approximately: 10 – 15W (incandescent) 0.6 – 1.8 W (LED).  

d) the switch must be accessible.  

e) rain light power supply may be separated from the motorcycle main wiring and battery.

they were using them at BSB last season

But we dont race in the rain or poor visibility anymore?

They might not start a race in rain or poor visibilty but with the Isle of Man weather you can never be sure there isn't either before the end of the race.
Sometimes at the Manx practice sessions I think it is getting close to the point where they need headlights never mind rear red lights!!!!

Very true. I think the point I was making was them saying they must be switched on when the race is declared wet when there will never be another wet race.

It can be set off knowing there is a visibility issue somewhere on the circuit with fog , and is this not a blanket ACU rule for all road racing , there for covering short circuit racing , where you do get wet races ??

All my comments are ref the TT.
It was my understanding that they were never going to start a race again when it is raining, or where the visability is poor, so other that it starting to rain or mist, fog coming down after the race has started, they wont be needed as the race is never going to be declared wet ever again.

This is for all tarmac racing, not just the TT or MGP.

I found them useful when catching slower riders in places like Glen Helen during evening practice, so I'm ok with this ruling.