A properly completed tiled bathroom does not leak and cracking a tile only happens if you drop something heavy onto it! As a result, waterproofing underneath a tub is not generally done unless it's something like a clawfoot tub. If the tub is installed level, the walls are properly waterproofed to the tiling flange, and you install the glass door properly, or use a shower curtain, you'll not get any water out of the tub except what you might drip when you exit it, which wouldn't rot out anything! In new construction, the tub is one of the first things installed, so having the apron below the finished floor is normal. It also can hide some minor imperfections of either the tub or the floor. You could install Ditra underneath it, but you'd want to fill it with thinset prior to installing the tub. In fact, some people do this on a regular basis as it somewhat protects the towers and makes layout lines easier to mark since the thinset will hold a marking pen better than the plastic Ditra material. The key to a long-term, care-free install, is proper prep, planning, and execution per industry standards. Unfortunately, getting all of those accomplished is getting harder, as any one oversight, intentional or accidental, can compromise the final result.