"They don't want sellers picking and choosing which of the available postage methods they are willing to use - to be fair, that would drag us back to the pre-SD days."

"Us" ?  

You're a business and can pick and choose the BEST option for yourself and your buyers from ALL the methods available, without ebay's intervention.  I think I'm right in saying that, if you want to, you can opt-in when it suits you and opt out again when it doesn't.