Funny, I'd been thinking about this very subject. Personally I would prefer to see them ride in *most* conditions with road legal treaded tyres for all. I've raced and practised in conditions they wouldn't run a race in now, and I can safely (literally!) say that it can be done as long as you bear the conditions in mind.

However, I can understand the factors that are driving them to the policies they have adopted:

1. Today's TT races are first and foremost a profitmaking show, not a trial or sporting event as they started out. Therefore the spectacle is what counts.
   
2. The greatest publicity value is achieved with ever-increasing lap speeds, meaning slicks.
   
3. The negative publicity value of a serious crash in the wet and possible insurance ramifications if the organisers were deemed to have allowed "unnecessary" risks by running in wet/mist.
   
4. Racing wets wouldn't last 2 laps, especially if the course was only partly wet.