I think it just adds clutter and confusion to the flow.
@TheDJ, can you expand on this a bit? What about the "Watch this page" do you think could cause confusion? And what kinds of people do you think could find this functionality confusing?
One thought that comes to mind: The verb to "Watch" isn't currently explained or defined anywhere in the interface, which could lead newer people to be uncertain about what affect the checkbox has, discouraging them from "checking" it.
Why do we need the 'watch page' option down there?...Are we targeting newbies or advanced users with that ? If the first, we should better explain what it does, if the second, do we really, really, really need it ?
Good question. The "Watch this page" is targeted for newer contributors.
Now, the question: Why is the "Watch this page" checkbox presented as it is?
In short, we think having the checkbox there will increase the likelihood newcomers will find out about replies to comments they post.
The above is informed by the fact that, by default, all newcomers across all wikis  will not have the "Add pages and files I edit to my watchlist" preference turned on.
Meaning, if the "Watch this page" element was to be excluded from the Replying workflow, there's a higher likelihood newer contributors could miss out on the input/guidance/help they came seeking.
I should note: "Watching" an entire talk page could become a distraction which is why, as @Pbsouthwood alluded to here, we intend to iterate on the "Watching" action to make it more "specific" with time.
Some examples of ideas we've started thinking about include: being able to watch specific sections and being able to elect to receive notifications when someone responds to a comment you post and/or a conversation you start.
If you have thoughts on this broader area of making it easier for people to stay informed about updates that are relevant to them and/or issues that prevent this from happening, I'd value you sharing them...here or on ticket T233447 in Phabricator both work.
1. There are two exceptions to the above: Arabic and Czech Wikipedias have overriden this default setting. Meaning, everyone has the "Add pages and files I edit to my watchlist" enabled by default.