Interesting, because I prefer the aggregator sites.

The aggregator sites usually contain some user reviews as well as a standardized UI - one I'm already familiar with - for booking a room. On the larger aggregators, I've also already got an account with some of my booking information saved. I also believe (perhaps irrationally) that buying through a large aggregator is more secure than buying through some random hotel site's booking solution.

If Google were to bump TripAdvisor in favor of direct links to hotel sites, Google would become less useful for me.

The same is true, as an aside, for restaurant websites vs. Yelp.