Well, after thinking more about the problem, one thing became apparent to me: If it was easy to write binary drivers that are compatible to more than just one kernel version, then hardware makers would probably starting to release Linux drivers in very large and uncontrolled quantities, which would

a) decrease quality
b) make it impossible for Knoppix to include them (nearly) all

So what initially seems like a problem (the impossibility of binary-compatible drivers) is perhaps actually an advantage of Linux?

But still, things like Nvidia need a solution.