You'd need a y to keep the cut edges parallel for slotting the cross pipe in.
If you rely on spinning the pipe to create the hole, the edges are always going to be half the cutter width of the centre line, creating an unequally chamfered hole. For small holes, or thin pipe, it's not too much of an issue, but if you're needing a tight fit, you really need to use a y-axis to keep all edges parallel.