Why does multidimensional arrays not allocate properly?

[Jot via Digitalmars-d-learn]

auto x = new int[][](n,m);

But one cannot freely assign anywhere in x:

x[3,6] = 4 crashes.

I, can, of course, convert everything to a linear matrix and 
index by i+w*j, but what's the point of having multidimensional 
matrices in D if they don't allocate them fully?