What your change suggests to me is that you'd encounter cases where `document_get_current()` doesn't return the document for which the signal was fired for. This sounds weird for the `activate`/`new`/`open`/`reload`/`filetype-set` signals you're using, and definitely something that should be fixed if it's indeed not in sync (e.g. the signal is fired before the changes that make `document_get_current()` return the right value happen) -- unless there is an actual reason for that, but still.
I'm sympathetic to the base idea of using the document for which the signal was fired though. But it really seems like it should be strictly equal to `document_get_current()`.