This topic is coming up every now and then. The challenge here is how to implement structural integrity without taking off from the building possibilities RW has to offer.
Some people are building not only houses, castles, etc. but also glasses, cups, shelves, tables, chairs and even pen, books, statues, etc. Not to mention floating castles, space ships, etc.
All this is possible because there is no SI / physics involved. Adding it would limit the possibilities we have.
Even if you could decativate it and activate only when required the activation would effect all built structures, not only the one you would like to apply it on.
I would vote against it as long as there isn´t a solution for that