I get the feeling this is not up for debate but...and maybe this should be moved to the suggestions forum. The updater idea to download maps/extras is a good idea.
People reading this thread and forum are the mainstay of the community and will probably download the maps. However, there are many newcomers to the game who do not regularly haunt these pages and they will in all probability not download the maps.
When adding these maps or just a few choice ones to a server list you will have new players disconnect as the new map comes up and they will almost certainly connect to another server which they can play on. The new map server will drop in population also discouraging other people to connect (may be even those who have the map)
Now I'm guessing the maps are being released because devs, creators and players want them played on? It does not seem like the best approach to achieving that goal.
The updater is a good idea or even a message when a user is disconnected saying be patient while the map they want to play downloads in the background.
Quake 1 did it back in the day (96 I think

) during the game you were playing, mods too (eg Thuunderwalker). Really convenient.
We do have 50 excellent maps but of course, more is...more. The main thing I think is that we all want all the releases and extra things that people work on so hard to be a success and be exposed to as many people as possible and auto downloads might help achieve that.
Spread the PR love, and Happy new year!
EDIT...
I think I was wrong, people have downloaded the map pack and there are full mappack servers...PRTA at least. Awesome.