To be fair, a builtin wardrobe is different to a walk-in wardrobe. The latter _tends_ to be builtin, the former tends be built into "the space between walls". I'm not 100% sure I would call them more space efficient, however they don't poke out into the room, and so you end up two rectangular rooms, and with their wardrobes next to each other, as opposed to two non-square spaces with the wardrobes (typically) backing onto each other. So you typically end up with extra usable space.

A walk in robe on the other hand typically wastes space since it (in effect) introduces a new hallway into your house, since that is effectively what the wakling area in your walk-in robe is.