The SymPy author was explicitly talking about users. The sort of folks that use matlab, mathematica, and maple.

I don't know of any of them that expect 2 / 3 to be an integer.

And as ufo says, it creates subtle bugs because it works right on most data until you just happen to get a pair of ints. Then boom.

eg in Matlab all numbers are doubles unless you take action to make them otherwise.