They get the tile straight by using a level and straight edge. Even tempered glass is not "super" strong and can be shattered by pressure in just the right location. Even if the sides are "frameless" the top and bottom should be stabilized somehow, especially if the door is going to be hinged off of one of the sides. Recessing the bottom into the tile gives good stabilization and as long as the joint does not crack should be leakproof. It does "lock" you into that enclosure however, unless you want to change the tile if you ever need a different style.