Baseplates can be used to support an upper level so long as there is sufficient support below. You generally need to support along the fours sides and, either (i) also support the middle or (ii) cover the baseplate in enough brick or plate to increase its rigidity to the point where it can bear whatever load you place on it without flexing.
Here is an example where I used baseplates to support the houses on the hill using Duplo and regular bricks underneath for support: https://www.flickr.com/photos/33202035@N06/8247374063/in/pool-steelcitylug/
I also did the same thing to support the floor of my Santa Building (there are four 32×32 baseplates making up the floor upon which the globe sits): https://www.flickr.com/photos/41914153@N03/10808965945/in/pool-steelcitylug/
And most of the cubes in the Dig Into Reading display from a few years back also used supported baseplates: https://www.flickr.com/photos/sixf00t4/8941828206/in/pool-steelcitylug/