1script - 4:10 am on Sep 13, 2012 (gmt 0)
You must be talking about good old pre-autocomplete times. Long tail keyphrases are the bread and butter of smaller niche sites, which Elsmarc above refers to as "Mom & Pops". Long tail is getting completely killed by autocomplete.
As you grow (in terms of visits) sometimes it's your head terms that grow, but most often it's long-tail. As that long-tail growth happens, your landing pages often become less effective because visitors don't see that the page matches their intent quite as quickly as they do when they arrive from a head term search.
I often have to search for esoteric very long-term phrases with various industrial part names and such - something that I used to feed the whole thing into the search box and Google used to bring just one or two results from the entire Net, but those were the results I needed. What happens now is that it is almost impossible to type more than 2 words in there - you're bound to either go on a tangent or hit Enter accidentally or a slower computer will play a trick on your and put a wrong suggestion in there JUST BEFORE you hit Enter - stuff like that. Yes, I do know you can turn autocompelte off, but how many people actually do that? Myself, I'm very frustrated by it and yet, incredibly, it's still ON in my FF. I imagine an occasional searcher simply does not care to turn it off or even has a clue it's possible.
So, anyway, going after long tail kws is all good but the long tail traffic has dried up significantly in the last couple years. Once you get there you may realize it was not worth the (little) effort.