Comment by shiandow
I don't think there's any mathematical reason to lay out the elements in memory that way. Sure given no context I would probably use i = row + n col as index, but it doesn't really matter much me.
If I had to pick between a matrix being a row of vectors or a column of covectors, I'd pick the latter. And M[i][j] should be the element in row i column j, which is nonnegotiable.
Matlab deliberately notes that its matrices are laid out like this since most matrix operations occur on columns, a whole column can be loaded on a cache line.