Kev999, I don't think there's any problem with the implementation as long as you intend to use it. It's kinda like routing a heap of tracks to a bus and lowering the fader on the bus.
Personally, I would like a red flag warning when I switch to the mode so I don't wind up in it unintentionally.
I'm not sure the best way to let me know which items have been modified using the offset mode. Perhaps when I touch the control it turns red? When I touch a related envelope?
I don't know. I used it correctly in the last couple of weeks and it is useful.