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Many astute advertisers know that parked domains convert two or three times better than Google or Yahoo search engine clicks.

The examples you gave are what give domain parking a bad name, however there are many REAL generic and expired domains that have excellent traffic like RumCakes.com or Computer.com - those are real and they convert for advertisers. Generally generic domains do the best, like the examples just given.

I'll tell you the reason they don't want to show parked domains on a one by one basis is b/c then you could cherrypick the domains which converted best for you as an advertiser and do deals direct. "Direct Navigation" traffic is the purest traffic there is and it converts really well, therefore it is also a threat to Google.  This is why it must be lumped together as a package.

It is estimated that parked domains make up 3% of Googles traffic.  But if it converts at double the rate then does that not make it about 6% of total conversions to sales if parked domains do this well.

Even Google's own internal tests have shown direct navigation domains to convert better than search.