I tell my customers that old tiles should be removed. We have very few jobs where the old tiles didnt easily fall away once you get started. Adhesives - especially some of the early trade stuff - dont have an infinite life. If one was to tile over a tile that seemed ok - but was perhaps only another few years away from failing, then the extra weight of the new tiles could compromise things. Also, plaster board can only have a certain weight of tiles on them. And in very old houses like down here in Portsmouth - the render is often only a few knocks away from falling away from the walls. Snot worth it.
