I'm not saying it wouldn't be handy. I'm saying I don't think the way Sigil's currently structured will easily allow that kind of "anything goes" save/open scenario. The vast majority of Sigil's feature set is predicated on xhtml being well-formed to begin with.
As I said, the only way I see around it is saving epubs in invalid states as a different filetype. Which while feasible, comes with it's own headaches and considerations. There simply is no "just save it as it is" solution to the problem. Never has been. It would be implemented already if there were.