I don't understand why people keep posting in this thread. In fact, I am locking it because I am that confused.
There are only two options:
1. Pokemon do not follow you (based on gameplay videos, and you can stuff the "BUT THEY COULD JUST BE BETA VIDEOS" argument, because if that was true they would have either a) revealed starter Pokemon first and had those following the player or b) released another gameplay video once starters have been revealed to show they follow you)
2. Pokemon can follow you, and it is optional (based on Celebi/Zorua events showing their sprites on the field, even though people seem to be forgetting that they are events and Pokemon have had overworld sprites since the beginning of time)
That's it. Where is the discussion here? Everyone is just talking in circles. Go away.