To be fair, I think the C2 TMX importer is a totally different workflow from Rex's. Rex's TMX Importer is a runtime importer, whereas C2's built-in one actually resolves TMX objects to C2 objects. This totally changes how you even design the game. I don't think it's terrible, but just a different workflow.
On the other hand, Rex's TMX Importer is comprehensive, allowing the reading of Tiles, Objects and Object-Groups. It reads Tile/Object 'custom properties' so it's possible to create a map with specific attributes within Tiled, and have that correspond to something in C2. The connection to the tmx is made during start of runtime, so if you build your C2 events and behaviours right, you can 'make' a game right out of Tiled by populating it with meaningful properties, and then using C2 as the interpretter of the tmx. Quite flexible, and frankly, quite fun.
Only rexrainbow can say, but it may be due to the C2 editor's limitations that his TMX Importer ended up being a runtime plugin. But I think it's brilliant implementation. I had my own reservations at the start, but eventually appreciated workflow of assigning C2 behaviours based on TMX properties. And the fact that you could only see the results after hitting the preview button.