-snip-
Which leads to the next question: Why would anyone choose to NOT opt in, as he would only miss out by not doing so. It´s all a mystery to me

Sorry for my ignorance
It's for tax purposes. If investors want to receive their XYM at launch they'd opt-in before the snapshot. If they want to receive them at a later date which is more tax advantageous, in a new tax year for example, they would opt-in at a later date.
This is if u store coins on a CEX. Like coinbase. And if whats yours are kept off exchange, taxation should not be a concern. I think I will go bust a nut and get back shortly to send a couple more word bullets in to the discussion. Its funny that no one posts here since no one generally shows any care toward the coin itself but when free token reins in or is distributed through fork, every penurious ass on earth with a wallet and a dust in it come here to ask for free crop.