For extra clarification. Even if:
1) All parts had all directional counterparts, and
2) There was a feature to flip the terrain 90 degrees,
then this feature still wouldn't be possible, because you would have to add these extra facings to the existing tilesets, and that would mess up map files. And even if you were prepared to redo hundreds of maps basically from scratch, you would run out of tile space (you can use 253 or so per terrain).
Having rotating maps would be nice, but it's not feasible even with a lot of work; unless you write a new engine with a new file format.