It is certainly correct that in any other industry you would focus on the 90% of sales not the 10% however when it comes to Adwords many marketer seems to completely overlook this in my experience.

I would disagree with your second point though. If you take a look at the search query data you will see that over 80% of the searches come from where users have typed less than 2 words in length. Only a small number of impressions are coming from searches that are 5 words or more in length so even if you had 1 million 5+ word keywords you would never be able to get the same number of impressions as you would with 2 word keywords if that makes sense.

By all means it is possible to manage accounts with huge numbers of keywords and we regally do at Clicteq through using things tools such as Adwords Editor and Adwords Scripts, however I would argue that for doing this with 5+ words keywords is probably not an effective use of your time