In-place installation brings massively more value to my organisation than clean install, though we would ideally like the option to run both.
Consider our situation as an application vendor. Let's say, for the sake of example, that our customer-base has millions of computers on which our product is installed, and perhaps thousands of new installations are made each day. If we look at the first day on which the cumulative update is released (and let's assume it reaches all of these computers at the start of that day to simplify the scenarios), 99.9% of our customers' computers (the millions) will go through the in-place installation scenario and 0.1% (the thousands) through clean install. The latter proportion will grow during the month, but to nowhere near the scale of the former.