What people want (at least some of them) is to know how much of their advertising spend in campaign X resulted in additional sales. That's quite hard to do simply with server log files since you need to correlate information from several sources. Of course, third parties could provide you with their log files that relate to your visitors and you could merge the two - but that would be no better from a privacy point of view than doing it all in the browser.
Personally, I think it was a great mistake to allow content in web pages from a third-party source - it's a privacy and security nightmare - and there should come a point in the future when it's disabled by default. After all, it's historically been assumed that your advertising spend is a bit of a lottery.