Well, you have so many fancy questions! Does anyone really talk about those things anymore in 2010, though? (Besides, how are they even relevant to readers of this site who might later search the words 'snow tires' and find your assertion that tread depth is not so critical for the Blizzak tires this OP asked us about?)
Up to date data on Blizzak technology is readily available on the
WWW. You totally miss the point of how those tires work when you simply and globally dismiss tread depth as an issue other than for hydroplane resistance.
First, you explicitly stated "On ice it [tread depth] doesn't matter since the contact patch is the same." This is just flat wrong with the tires our OP asked about with their microcell construction on the outer 55% of the tread. This construction, of course, is likely why you observe that Blizzaks wear out faster than other winter tires. So, to help you update your familiarity with this tire line from 15 years ago - and I only speak as a consumer of these tires and other brands, not as a tire engineer - here are a few current and relevant Blizzak points from just one merchant source:
On Blizzak specifically:
http://www.tirerack.com/winter/tech/techpage.jsp?techid=116
and
http://www.tirerack.com/tires/tires...eModel=Blizzak+WS60&PID=3724396&AID=10398365&
and on tread depth: I guess you want to see this -- that is, unless your tire testing lab has already developed its own top-secret answers ;-)
http://www.tirerack.com/winter/tech/techpage.jsp?techid=163
and my personal favorite related to tread depth:
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=157
Of course, this is all just one source (even if the leading one) directed at consumers like me and not fancy talking tire showoffs or anything like that. You could find other sources yourself - or, perhaps, simply dismiss this altogether. Others might form their own opinions on Blizzaks from easily accessed current material, as I suggested above. I researched them and formed my own opinions after owning many sets as well as Nokian and other winter tires over the years, too. Luckily, though, I don't live in the Arctic where folks seem to take even polite disagreement so very, very hard . . .
Your original note above said nothing about "other tires" just your opinion about all winter tires' tread depth and water/slush. Future searchers here about the OP's Blizzaks need more facts about that specific tire, I believe. If you disagree, go pound some sand - or snow, LOL!