Well, a ranger can have a total of 7 advantages, one innate, and three from each tree. But using any advantage puts it on a 60-second cooldown and all six other ones on the same 35-second cooldown. So in reality, you will most likely rotate only between two of them. The only reason I could ever see for giving so many options and then extremely limiting their use would be to give the ranger a bit of a choice to customize his or her play style. You might go with a shattering/fire build, for example, or choose to feat either shattering or fire and pair it with the innate advantage, focused fire. But there would hardly ever be a need to feat even the four one-point advantages, let alone the two multi-point ones.
I never much liked this arrangement, since I feel it cheats the ranger out of some pretty nice options. Imagine, for example, if bear shaman's manifestations were set up in the same way.
At the very least, I think there should only be a 20-second cooldown for the unused advantages so that it would be possible to rotate among three of them rather then just two.