In short, if Pike could have talked, the "mystery" as to why Spock would seemingly go AWOL is negated. In short, it's the "McGuffin", a motivating factor for the narrative. Or, if one is cynical, it's a plot contrivance. Maybe the nature of the Delta ray damage made normal interfacing considerably more problematic.
Out of universe, of course it's a plot contrivance. I think Melakon
was likely trying to fit in the events of the episode with what else we've seen done.
As far as Next Gen
goes, I thought Betazed was one of the Federations founding members? Though I could be mis-remembering that part. EDIT: Definitely wrong on this point. Memory Alpha gives a 2273 join date.