"I come back to you now, at the turn of the tide"
I think they had to show Gandalf showing up.
Somehow, most people would have gotten a clue anyway, either by careless references or by the smug grin of those who read the book.
Either way, many people read the book after seeing the movie, so they'd know too.
Additionally, Ian McKellen probably has a billing contract, so they'd have to include him in the titles, and the promotion - so they can spell it out as well.
Berandor