The problem isn't just SEO, it's that Google itself aggressively rewrites queries to produce more results (which I suspect they want to do to show more "relevant" ads).
On the most extreme end of this, I've seen four-word queries produce results, in which three of the words were stricken out. More often, it's just one word, but it's usually exactly the one that makes the difference between a very specific query, and a very generic one.
Worse yet is that they try to do synonym substitution, but their algorithm has a ridiculously low bar for that. Like, you might be searching for "FreeBSD", and it will substitute that for "Linux", or even "Ubuntu". Or search for a specific firearm model, and it finds "gun".
Quoting keywords suppresses all of that, but synonyms are actually useful - if it did them accurately...