Going back to first principals, the original concept of the TT (remember Tourist Trophy) was to test, compare and prove motor cycles under the most severe and difficult "road" conditions. (Off road racing being a different concept).
During the 53 that I have been riding powered two wheelers for getting to work, fun, "touring", pilgrimages to the IoM, etc, I never had a choice of what the weather may be.
As has been said, the wet is a great leveller where the big bucks of high horsepower are less effective and more is down to rider skills and safe handling of the bikes.
I understand that the insurers won't allow racing on the Mountain Course if visibility precludes safe helicopter use but otherwise, I feel that racing should continue on wet roads with, of course, suitable tyres.
I've said it before (Sorry Malcolm) but I still believe that we should use different warning flags for oil and water.