I don't really think it would feel like a valid replacement for a world editor... but it would definitely be useful and a nice feature to have. I'd like a way to add additional bulldozed versions of existing neighbourhoods, so I could make an extended Willow Creek, for example. I find I use a lot of lots in certain areas and run out of room for everything I want to build with that theme. So, the idea does appeal to me.
Something like TS2 where you choose to add more pre-built or empty versions of maps would be useful.
