Hi Gabriel,
Thanks for sharing it, I played with it a bit and had some fun making cages! :)
I like how it manages to connect different pipes very well.

I have some suggestions that might be useful to mention for improving your command.

1- Your command supports one radius for all pipes. Also, It only uses the low poly cage for the sub-D importer.
I think having an option to output just the subD cage would be useful, so it's possible to modify and connect multiple pipes with different radii. and then do the last part (sub-D import) manually or via a script.
Or even to make the preview faster, having an option to skip the sub-D import and show the raw cage on the viewport.

2- I tried it with >100 curves, some in very tight angles, as expected it failed on some of them. The command ui filled with info about the coordinates of failed curves ("The joint at (x,y,z) could not be built"), almost breaked the ui. I think it'd be much easier to track the bad curves to display them on the viewport rather than displaying the coordinates. Selecting/highlighting the problematic curves is a simple solution for that.

3- This one is not that important, but a "Back" button might be useful when doing some trial and error to get the best result without canceling and running the command again and again.

Thanks