Its no good telling Apple (for instance), when they reject your book, that their validation procedure is wrong and your epub is right because it passed the lame validation in Sigil.
Because of course, that's exactly what I suggested you do.
The point you're
missing is that if the various distributors are going to start cherry-picking what's appropriate and what's not in their particular flavor/mutation of epub, then they need to have their own publishing software that ensures
compliance ... and commercial creators probably need to use it (if they don't want to get their hands dirty tweaking Sigil's output for all the various distributors, that is). Heck they may even have to pay for it.
It's silly to think that Sigil is ever going to be able keep track of all the ever-changing idiosyncrasies that the various distributors are introducing into their submission demands. They'd never get anything else done. And I'm sure the fragmentation is only going to get worse before it ever gets better.
So while I can sympathize with your plight, I don't think expecting an open-source software project to voluntarily choose to morph into a be-all-end-all validator for hordes of fragmented ePub-ish commercial distributors to bail you out of that plight is the way to go.
The whole point of Sigil is its a dedicated epub editor that is supposed to save time and produce distribution-ready books without having to faff around.
Hmmm... did I miss the distribution-ready
part of Sigil's mission statement?