Numpy is good for vectorized calculations on dense multidimensional arrays of homogenous 64 or 32 bit floating point numbers, or integers. If that describes your problem and you like Python, by all means use numpy. If that doesn't describe your entire problem, you'll need more tools.

It should be understood that numpy is written in C, and is an example of the "2 language problem" that Julia solves rather well.

The question really is the converse ... given how deft Julia is with general programming as well as numerically intensive programming, do you really need Python + numpy(C).

Numpy is great as long as you're using Numpy. There's lots of aspects of scientific computing that may not be using Numpy.

On top of what others have mentioned, there is something to be said about getting numerical linear algebra tools as native citizens of a language.

There are many things I hate about MATLAB, having matrices built into the syntax is nice.