One thing you can be sure of - Google makes changes like this with one purpose - to meet the average searcher's need. They measure the data that's generated (and it is a LOT of data) and if the measured user response is not good, they will back out of it. They don't really care about anything else but the user data, including predictability of their results. Google has been a disruptive force from their very beginning - a disruptive force in a new and very disruptive medium.
So what's interesting to me is that they've clearly been building algorithms around when and how much synonyms should be folded into the search results. from some of these examples, it looks like multi-word queries stand a better chance, possibly because strong content on the exact matches is harder to come by, and strong content on synonyms is more likely to meet the average user need.
I don't think the average user much cares whether "leather couches" also returns "leather sofas" - in fact, I'll bet they do appreciate the synonym results.
Ever since Google went into semantics well over five years ago, it's been a wise web author than loosened up on the tight "keyword match" content writing.