At Interbike, I posed that very question to the person who is in charge of pretty much every tire that comes out of the Schwalbe factory, and his response was much the same as that provided by Sheldon Brown:

"Unfortunately, most people assume that a smooth tire will be slippery, so this type of tire is difficult to sell to unsophisticated cyclists. Most tire makers cater to this by putting a very fine pattern on their tires, mainly for cosmetic and marketing reasons." SB

I asked this question because I was puzzled by the tread pattern that had been designed into our road racing rain tire (Ultremo Aqua). Surely I thought if there ever were a "sophisticated" segment of the market, this would be it, and no tread pattern would be introduced to a tire that surely should not have one. The same logic was given. So, all I can do is buy my slick Ultremo tires and hope that some day we'll all be able to reconcile what we know with what is offerred in the marketplace.