The primary usefulness I would see is syncing your browser history, bookmarks, etc. across devices. I use Safari and I have pretty much every extension I feel like I need.
Not sure I understand your question, but the thing I'm talking about is a new "Firefox for iOS" product being worked on right now, due for release pretty soon IIUC, that will include syncing among its core capabilities:
I wonder if there could be some collaboration where firefox, chrome, safari, IE, opera, etc. can exchange and sync - possibly too much to deal with, and might hurt innovation.