My point is that if one is not comfortable with basic concepts in Math (regardless of major) then it is likely that one will not do well in this test. But this test does not measure how well one can grasp abstract ideas and prove results which is what one does in a Math PhD and what we did as undergraduates in Math. I personally don't care for the test and really hope that universities use this piece of information (GRE scores) of our apps with caution.
In a nutshell, I personally don't think that someone who scores in 80 percentile or higher is more likely to be a better mathematician than one who scores in the 60 percentile. It just means that he is a more pragmatic, swift and precise problem solver. However, if one scores percentile 30 or lower than it is pretty clear that this person does not really know his math (especially considering that 50% of the problems are of Calculus).