I agree that there will be many "builds" that don't get much attention.
I agree that not all spells will be useful in the end game.
That said, if you where to choose 3 skills you really liked and wanted to max out, and then choose 4 more from whats left to use as supporting skills, then there are 16.8 million possible combinations per class. The actual number of points you put into each skill is irrelevant.
The point is those formulas consider a build with 14 in teleport and a build with 15 in teleport as two different builds, but they would play nearly identical. If you're looking at builds that actually play differently, each class probably has high hundreds, maybe low thousands. It's still tons of customization with mind blowing variety, but it won't be millions or billions of builds.