Because of the rounded profile of bike tyres, tread compound plays just as big a part, if not bigger, regarding grip in the wet as the tread pattern.
This was a theory of a certain Mr John Robinson of PB fame and was put to the test many years ago by Performance Bikes on a skid pan. The pan was filled to about an inch of water and then they fitted a number of different pre-warmed tyres to a BMW K100 (about the only bike back then with ABS) and tried to brake as hard as they could.
What they found was that the sportier tyres with less tread pattern out braked the harder compound touring tyres. So to properly test the theory they fully warmed up a racing slick and then tried the same with this to find it out braked all the others by a considerable margin!
Of course this test, done with tyre warmers, didn't take into account a sport touring tyre's ability to reach temperature quickly and then more importantly, maintain it without overheating.
However as the tread wears down it is less able to distort and spread around the contact patch and as such is less able to grip the surface than a new tyre. And as such a worn tyre suffers as much from the lack of tread material as it does from the reduced depth of the tread grooves.
Of course the tread pattern does still play an important part in displacing water. However, although I can understand people's concerns over the lack of tread pattern on the edge of modern tyres, because a tyre has a tendency to squeeze water out towards the centre when learnt over, there is less need for tread pattern at the edges than in the centre.
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