I just happened across that "daily 2PM to whatever" thread right when it was opened (by someone specialising in Build Mode, as it happens! =) -- well, now I asked that same question twice (about the floors/planes/stairs/stuff). Information overload! They said essentially the same though: foundations now are intelligent, telling the sims "you can walk down here".
I have to say I am
slightly envious of your world builders. XD
Okay, I will pretend the basement conversation never happened. =P
(CFE is useful for pretty much everything, basements too! Like when you want to build L- or U-shaped stairs that lead down to "DIY" basements (not the ones made with the basement tool), you need cfe to slope the walls around them.
Like this.)