Completely agree with you. There is no correlation between F1 performance and the character and quality of streetcars, though some marketing gurus claim that carmakers may benefit from their F1 experience by adapting some technical innovations for their streetcars. I doubt this and I don't know any P-car owner who wants to have a HANS device fitted into his car.
I guess those of the big carmakers participating in F1 carve the F1-budget just out of their huge marketing budgets. Remember Porsche does not have a big marketing budget (in comparision to the biggies) - almost no TV-commercials (at least in Europe) and very few adverts in magazines/newspapers. As they say in the corporate world: half of any marketing budget is useless, but you never know which half...
As a Porsche-fan and F1-fan I'd love to see Porsche join the party, but financially it doesn't make sense and the Weissach-guys don't need F1 to come up with technical innovations for the streetcars (see VTG-technolgy for the 997 TT).
BTW Renault did almost nothing to use their 2005 F1-championship as a marketing-tool for their streetcars (almost no coverage in their regular adverts etc.).