to make it more real/reliable/accurate, everyone would have to have the addon. the addon would be spamming addon messages left and right about nearly everything... ya.. it get complicated and laggy real fast..
I don't think this is neccesarily true anymore. TARGET_CHANGED fires for *every* raid member I believe, so you'd be able to pick up when people target you. If their target is your target, you can assume they are asssiting you. Also I know Pitbull can have cast bars for every since raid unit frame, which leads me to believe pulling casting and spell information is also possible (without others using said mod).
Complicated...yes, but I believe it can be done (and done locally, without making others run something)