I foresee this feature being a useless gimmick. What advantages does building such things have?
Useless for most of us harcore PC users sure, and certainly anyone who can use the GECK...since we have access to innumerable house and community mods; BUT not so much for consolites who actually have played FO for years with zero modding capability.
TH already said you don't need to use the feature to traverse the game, but this mechanic at least gives console gamers a taste of an in-game toolkit. I think you underestimate how popular minecraft type gaming is with the console crowd. It's one of the few games that gives them any notion of freedom outside the strict boundaries set by modern day developers. Not to mention the busy time it will fill for PC players who will just plain like to do minecraft type tinkering...and I don't think that's as small a crowd as many believe. My teenage kids actually can't wait for this feature of FO4 even though their old man brushes it off in favor of getting his hands on the new GECK.
I predict a ton of photos on Bethsoft showing each players new massive creations.
Bottom line: useless to some; not so much to others.