I have IRC logs, so let me just paste the whole conversation - the one I was thinking of when I posted that above. TheDanW is the UI dev. This conversation occurred in December 2017, after BFA alpha was out but when addons were disabled so most of us were just watching the interface files for diffs in order to speculate on what changes needed to be made.
https://paste2.org/wZGNWyGv
There's at least a few more instances of this type of conversation, but I don't really want to dig up his entire post history for people to take apart outside context, just to post a bit so you can see Blizzard's reasoning on this. Also, he mentions in one other conversation that if people are interested, he could add support to a few frequently called functions so that somebody can pass their own table as an argument in order to be reused. Might be a decent thing for somebody to suggest for GetPlayerMapPosition, though I'm not sure how viable it is due to it being a mixin (Vector2D) return.
P.S. I realize "Dec 18 17:43:56 <Simca> all new API since WoD return tables, with a few exceptions" this is quite hyperbolic. There are certainly a large number of tables returned by API functions since WoD, but there are more than 'a few exceptions'. Not sure why I decided to say it like that in retrospect.
Edit: Also, he mentions elsewhere that the optimizations for the garbage collector he is talking about were changes that went Live with Patch 7.0.3.