> now you have to add a check for null inside your function

While I generally agree that non-const references make more sense for out-parameters (although I actually prefer return values), don't forget that it's possible for a reference to become "null" too. It's quite unlikely to happen, but if it happens it's much worse to debug than a null pointer.

Oversimplified example:

    const int& bla = *((int*)nullptr);