I'll give that a "maybe". Rain is not a great lubricant. The only really slick stuff when wet seems to be uber smooth tar patches. So yes, if you are accelerating out of a corner at WOT in the rain TCS would probably help. But if you're accelerating at WOT out of a corner in the rain... Maybe I'm just getting to old. LOL
I am not endorsing WOT out of rainy corners, it was just an example though likely a common occurrence in racing.
I'd have to disagree about rain's effects, though. In my experience traction can vary greatly across different road surfaces, and spinning the tire during acceleration can occur at above 1/2-3/4 throttle applications on certain road types.