The whole point was to mimic a real Sky HD bouquet including numbering, importing cable channels where necessary. If you use a "Custom" region you would end up with SD numbering.
Also if you look at the history of ABM it goes like this:
- Add "extra services".
- Add "access to bouquet key for conditional serviceshacks".
- Add ""swapping between providers".
So previously the only possible way to import RoI channels was a combination of "extras" and "conditional servicehacks". But now there is "swapping between providers" you are no longer limited as to which bouquets those channels are imported into.