Some custom fields we end up reusing between Master Features, Features, Requirements, either naturally or because we don't want to lose data as items get promoted or demoted. As such we find ourselves have to manually replicate lists and manually reenter the data multiple times. Being able to reuse lists or copy field definitions between layouts would solve this.
Also, Custom Tables aren't available to non-Enterprise+ customers. So this workaround isn't available to most users.
From what I can tell of custom tables, this seems like a very complex way to workaround the original request.
What I need (and what I believe Wes is describing) is the ability to associate a custom field with multiple types of objects, just like in Jira where a custom field can be applied to Stories, Epics, and other custom issue types with a single field configuration.
This is exactly what custom tables are for. You can create a custom table and share it across any different record type. The other benefit of custom tables is that you can store more information than just the textual name of the item.