Hans, you seem to want some kind of continuous save, what would be the only solution to keep your work as it was at the moment of a crash, since it may happen at any time! [:)]
More seriously, I'm ok to program an autosave feature, because it's now possible to save a project in Sweet Home 3D without stopping user input. But I don't like the backup file option. Creating a backup file has as much chance to crash than saving the project itself.