That's why I say, it's viable/efficient if it will be added to the game officially and Valve can set it up that way (props outside of the map to avoid duplication), but it's not viable before that happens for anyone to actually run these maps on community servers. Though I dunno how Valve chooses maps exactly.
Frontline is the first community project I ever actually decided to look at, though invasion was the first one I heard about. So I dunno how stuff was done before, though this one does seem more organized and advertised.
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Map file size isn't a problem for community servers (non-valve servers hosted by whoever) unless it's a single map whose download is too big. I run one server with 100+ maps (VSH) and it's really only a problem for anyone if a single map's size is over around 40-60mb or something, which tends to be big for a single map.
I think the multi-map optimization is only more important if a set of maps is to be released officially as part of the game for everyone who has the game installed, as opposed to the portion who'll join community servers to try other stuff out ;p
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I'm just not sure how people were planning to playtest their maps with them set up that way.