How are Qubits useful?
So you can't find out everything about the value of a qubit. That doesn't mean you know nothing about it. And, sure, quantum computing probably would be more powerful if you could find out everything about the value. But, again, the fact that you can't do everything doesn't mean that you can't do anything.

In particular, you know that the measured value of a qubit drawn from a probability distribution constructed by some quantum algorithm. The algorithm will, presumably, construct a distribution that's quite tightly concentrated about the true answer. And you can always repeat the computation to get multiple samples.