Ben
From what I've read the key point of the Doak scale is to discern the difference between the good, the great, the exceptional, and the best. If 8s, 9s, and 10s aren't given out sparingly and very delibertely the scale ceases to be effective. Anyway i don't believe that the authors are limiting the number of 8s, 9s, and 10s to any certain number of courses, it just so happens that what they believe the top 100-150 courses happen to fall between those three numbers.
While a 7/10 may not seem like a great score right of the bat, it really is. In my experience a 6 is a pretty darn good course, and a 7 is a really good course. Just going by Tom's numbers Mid Pines received the same score as The Ocean Course, Yeamans, Pasatiempo, Cal Club, Harbour Town, Old Town, Peachtree, Kapalua, and many other great courses.
Not to mention that Darius gives a 7 to Machrihanish which he currently has #43 on his world top 100 list.
A 7 is generally nothing to complain about.