As a "dabbler" in Sigil, my main use of it has been for correcting blatant errors in texts (spelling, punctuation, etc.), rather than structural editing (for want of a better phrase). A WYSISYG interface is brilliant for that purpose, as it hides all the code, etc that I don't want to change!
With regard to the issue of validity of epubs, I agree that Sigil shouldn't enforce validity at all times - but please provide a nice, easily accessible way of checking validity that doesn't require processing the file in a separate application (sounds like a good job for a plugin?).
with regard to the issue of making Sigil non-cross platform - I'm a Linux user, so you can probably guess my position
Good luck - whatever direction you end up taking Sigil.