Custom attributes can be modified via complicated scripting methods, however I think that if I need to sort my devices, being to bulk change these attrubtes from the UMS would be a huge benefit.
I think you should be able to bulk edit these for a view, selection, folder etc. For example: Search up all the devices on an IP range in a view, and then right click the view and select a change Attribute and set the custom attributes from a dialogue.
I think if these devices are going to be managed by the UMS, sometimes, or most often or most critically, what the UMS considers for these attributes is most important.