It's an interesting question about tracking many vs. a specific group of key terms.
The main reason for multiple terms, I would say is that we truly don't know what google's doing on the back end (or rather don't know every association they make), and so we may see how one term jumping up may also affect another term. There's so many permutations of each phrase:
dallas seo, seo in dallas, seo dallas, dallas seo firm (+ reverse), dallas seo company (+ reverse), etc.
While I'm not exactly keen on knowing all permutations of every single phrase, it's interesting how google gives us credit for one and not the other, but we won't know unless we're either tracking it or it pulls into our GA account.
With that said, you're probably right about the KPI's, but you might not find a phrase's potential without knowing that you're right on the cusp of bringing it up to page 1 for a client or yourself. If you do know that, you can make the efforts to pull it into the prized 3 positions where there is actually traffic and then see if it does pull traffic for a client.
I don't know if I'm answering or clarifying my stance really on the multiple phrases that well, but the ultimate point is that sometimes being able to see factors on phrases you don't keep close tabs on every day can be interesting in a) learning more about the algo's correlation between phrases and b) whether you're close to getting a phrase somewhere where it might mean something to you in terms of traffic, etc.
Then again, how many people can regularly process thousands of keywords and put the connections together in their head? Right?