I think you miss he point.
The usefulness of a per-user block list is not fighting outright spam. The statistical filters, etc, as applied by the search engine, work best here, as you correctly state.
The point is to remove certain well-known sites with information about well-known topics, which is unhelpful for the particular user, me. That information is technically not spam, but it's on a wrong level, badly presented, etc.
That is, if I search some info about a particular CSS trick, I want MDN be on top, some professional blogs be near top, and w3schools be on the third page. For someone else, priorities may be different, but I know what to expect for the search queries I issue, and I'd be glad if the search engine helped me.
Equally, certain domains are known to be more rich with information I happen to seek, like reddit.com, so I'd like to give them a boost. Some other people may prefer something else, or not give any boost at all.
So, this is personalization, not anti-spam. If Google kept it, it could even bring important insights into what ads I would skip and what would click :)
Maybe 90% of users would ignore this feature. But the 10% of power users maybe disproportionately important: with more disposable income, more clout on social networks, more decision-making power. I suppose that the shutdown of Google Reader, which affected a relatively small number of users, affected all the wrong people, and destroyed so much goodwill that the losses from it are many orders of magnitude larger than whatever savings from not supporting it any more.