While doing the 24h Le Mans I really did need rain tyres. When it started raining I could just keep using racing soft but I noticed they started losing more and more grip every lap. Then I switched to rain tyres and I had some more grip again (though rain tyres don't really give much grip either). I think it just has to rain for a while before you really notice the grip on racing soft. Was raining for around 5 laps already before they actually started losing grip.
Haven't really tried racing soft in the rain on other races though, maybe it has something to do with tyre wear/fuel depletion being on or off as well.