I am trying to understand exactly how HDAs should be used in Unity.
Until now I have treated them as only having the lifetime of the operation they perform, but for example, if you make a road with an HDA, you would want to be able to edit it later down the line.

Is the intention that HDAs can be left around in a scene to be edited later?
Is there a cost, scene load and performance wise to doing this? Should I be able to have 100 road HDAs around?

Yes, you can leave an HDA in the scene indefinitely (or as many as you want). At build time, HDA meta data is automatically stripped out, leaving just the Unity components and data (e.g. mesh, materials, your own scripts, etc). No cost at all since at build time the meta data is stripped out and the plugin is not used.

In terms of workflow, the plugin supports both types: leave HDA in scene, or remove it after baking. Even after baking you can reinstance the HDA, and updated the baked gameobject. You can also save the HDA's parameter preset to a file and check into your source control and use that to reinstance another HDA later.

Note that an HDA is only “active” when a user starts changing the parameters. Similarly, the Houdini Engine plugin is only activated when an HDA is being used (changing parameters or cooking).

Perfect, glad to have had that clarified. Thanks!