It is completely valid. It's just that in large objects, there is often a kind of "averaging" effect taking place that results in the (much more complicated) QM solution giving more or less the same thing as good old classical mechanics. So you could in principle just use quantum mechanics to predict the course of a ball that is thrown into the sky, but that would be much more complicated for basically no reason and it would give you more or less the same result as just doing the simple classical calculation. It is similar to how you don't use special relativity for the same problem. It's valid, it's just pointless.
Of course, this is not the case for other problems, in many cases effects specific to quantum mechanics are important for the examination of large objects.