Oh you can still make errors in PageEdit
but once you exit PageEdit you are faced with CodeView in which you can clean up any messes you made, run epubcheck, force yourself to learn some xhtml, etc, etc.
Too many people used BookView as a crutch and never once looked at the actual code generated and old Sigil let them get away from it. This made many users feel that any mistakes they made were the responsibility of the Sigil Developers who could really do nothing about it.
Separation was key to dealing with that and forcing users to take responsibility for their own code if they choose to use PageEdit to do more than simple typos, proof reading, and basics.